Summary
---

We would like to be able to provide chain-specific default mined block
sizes. This will allow us to fine-tune the defaults that BCHN ships with
to be specific to each chain. Motivation: What is a good default for
ScaleNet or RegTest doesn't necessarily work as a global default for
MainNet.

This commit removes the global constant `DEFAULT_MAX_GENERATED_BLOCK_SIZE`
since its existence now could potentially be a source of confusion and
bugs.  The default mined block size now is a chain-specific setting that
lives in `Consensus::Params` and is set-up in `chainparams.cpp` for each
of the 5 chains we support.

Moreover, the `Config` object manages the user setting as specified in
config file or CLI.  Two new methods were added to the `Config` object to
get/set the `GeneratedMaxBlockSize`.

As such, we got rid of checking the `gArgs` object for `-blockmaxsize`
during critical paths, since that is just wasted CPU cycles (the object
must re-parse the string arg each time!).

`init.cpp` is responsible for setting up `Config` to respect the
chain-specific defaults, as well as for reading in the user-specified
setting `-blockmaxsize`.

Unit tests were updated, as was `miner.cpp` to use the new config value
(rather than read `gArgs` each time).

Invariants are enforced within the `Config` object on
`SetMaxGeneratedBlckSize`, (as was already being done with
with `ExcessiveBlockSize`).

Also in this commit: Fixed the -hh help to show chain-specifiec
-excessiveblocksize and -blockmaxsize.

Test Plan
---

- `ninja check check-functional`
- Verify `bitcoind -hh` has correct chain-specific defaults in the help
  text.