- add get_block_cache_usage/1
- add block_cache_capacity/{1, 0}
- add new db option {env, Env} where Env can be `default` or `memenv`.
- add new db option {db_write_buffer_size, INT}
- fix: cache usahe, correctly reuse the cache among instances
- fix: options parsing, reuse code where we can and correctly
handle db optinos

*** BREAKING CHANGE ***

The following cache functions has been removed:

-  new_lru_cache/1,
-  new_clock_cache/1,
-  get_usage/1,
-  get_pinned_usage/1,
-  set_capacity/2,
-  get_capacity/1

instead use the new block_cache function to change the
size of the shared cache. A new api to setup different caches
will come in the next version.

The following functions has been removed:

- default_env/0,
- mem_env/0,
- set_background_threads/2, set_background_threads/3,
- destroy_env/1]).

For now it's not possible to create a shared environment. also
the {in_memory, true} setting has been removed.

To set a memory environement use the option {env, memenv}.