Allow clients to set maximum number of memory mapped files

Original [issue 232](https://code.google.com/p/leveldb/issues/detail?id=232) created by mattjrichards on 2014-04-16T18:12:59.000Z:

Currently LevelDB sets a maximum of 1000 mmap()ed files on 64-bit systems and none on 32-bit systems via the MmapLimiter class. It would be beneficial for clients to be able to control this limit. For example, a client concerned about reliability might set this limit to zero on all systems to avoid SIGBUS when reading mmaped() files that have unrecoverable read errors.

Admittedly the notion of memory mapped files is hidden in the POSIX environment utility class, but I think they are a common enough feature that it wouldn't be to bad of an abstraction leakage to expose a setter for this limit.