correct size of default memory map

[mykmelez]

The LMDB docs for mdb_env_set_mapsize says that the default map size is 10,485,760 bytes, i.e. 10MiB. But the DEFAULT_MAPSIZE define in the LMDB code sets the default map size to 1,048,576 bytes, i.e. 1MiB.

I submitted a patch to fix the LMDB docs in issue 8322. That issue has been open for several years, and it isn't clear whether it's the docs or the DEFAULT_MAPSIZE define that is incorrect. But I suspect the former, since the comment above DEFAULT_MAPSIZE says it's "certainly too small for any actual applications," which suggests that the smaller size was intentional.

In any case, it's worth fixing the lmdb-rs docs to specify the correct value (even if the LMDB docs specify an incorrect one).