Provide mdbx_env_set/get_option()

[erthink]

Provide the set/get functions for a various options at runtime:

• the maximum number of named databases for the environment;
• the maximum number of threads/reader slots;
• threshold (since the last unsteady commit) to force flush the data buffers to disk;
• relative period (since the last unsteady commit) to force flush the data buffers to disk;
• limit to grow a list of reclaimed/recycled page's numbers for finding a sequence of contiguous pages for large data items;
• limit to grow a cache of dirty pages for reuse in the current transaction;
• limit of a pre-allocated memory items for dirty pages;
• limit of dirty pages for a write transaction;
• initial allocation size for dirty pages list of a write transaction;
• maximal part of the dirty pages may be spilled when necessary;
• minimal part of the dirty pages should be spilled when necessary;
• how much of the parent transaction dirty pages will be spilled while start each child transaction;
• etc?