This modifies the utxoset in the database and related UtxoViewpoint to
store and work with unspent transaction outputs on a per-output basis
instead of at a transaction level.  This was inspired by similar recent
changes in Bitcoin Core.

The primary motivation is to simplify the code, pave the way for a
utxo cache, and generally focus on optimizing runtime performance.

The tradeoff is that this approach does somewhat increase the size of
the serialized utxoset since it means that the transaction hash is
duplicated for each output as a part of the key and some additional
details such as whether the containing transaction is a coinbase and the
block height it was a part of are duplicated in each output.

However, in practice, the size difference isn't all that large, disk
space is relatively cheap, certainly cheaper than memory, and it is much
more important to provide more efficient runtime operation since that is
the ultimate purpose of the daemon.

While performing this conversion, it also simplifies the code to remove
the transaction version information from the utxoset as well as the
spend journal.  The logic for only serializing it under certain
circumstances is complicated and it isn't actually used anywhere aside
from the gettxout RPC where it also isn't used by anything important
either.  Consequently, this also removes the version field of the
gettxout RPC result.

The utxos in the database are automatically migrated to the new format
with this commit and it is possible to interrupt and resume the
migration process.

Finally, it also updates the tests for the new format and adds a new
function to the tests to convert the old test data to the new format for
convenience.  The data has already been converted and updated in the
commit.

An overview of the changes are as follows:

- Remove transaction version from both spent and unspent output entries
  - Update utxo serialization format to exclude the version
  - Modify the spend journal serialization format
    - The old version field is now reserved and always stores zero and
      ignores it when reading
    - This allows old entries to be used by new code without having to
      migrate the entire spend journal
  - Remove version field from gettxout RPC result
- Convert UtxoEntry to represent a specific utxo instead of a
  transaction with all remaining utxos
  - Optimize for memory usage with an eye towards a utxo cache
    - Combine details such as whether the txout was contained in a
      coinbase, is spent, and is modified into a single packed field of
      bit flags
    - Align entry fields to eliminate extra padding since ultimately
      there will be a lot of these in memory
    - Introduce a free list for serializing an outpoint to the database
      key format to significantly reduce pressure on the GC
  - Update all related functions that previously dealt with transaction
    hashes to accept outpoints instead
  - Update all callers accordingly
  - Only add individually requested outputs from the mempool when
    constructing a mempool view
- Modify the spend journal to always store the block height and coinbase
  information with every spent txout
  - Introduce code to handle fetching the missing information from
    another utxo from the same transaction in the event an old style
    entry is encountered
    - Make use of a database cursor with seek to do this much more
      efficiently than testing every possible output
- Always decompress data loaded from the database now that a utxo entry
  only consists of a specific output
- Introduce upgrade code to migrate the utxo set to the new format
  - Store versions of the utxoset and spend journal buckets
  - Allow migration process to be interrupted and resumed
- Update all tests to expect the correct encodings, remove tests that no
  longer apply, and add new ones for the new expected behavior
  - Convert old tests for the legacy utxo format deserialization code to
    test the new function that is used during upgrade
  - Update the utxostore test data and add function that was used to
    convert it
- Introduce a few new functions on UtxoViewpoint
  - AddTxOut for adding an individual txout versus all of them
  - addTxOut to handle the common code between the new AddTxOut and
    existing AddTxOuts
  - RemoveEntry for removing an individual txout
  - fetchEntryByHash for fetching any remaining utxo for a given
    transaction hash