Mmap size option

This pull requests add the mmap_size option to the options passable to the levedb instance. This is a more general approach to the developer_limit_memory flag which gives the developer control over how much space mmaped files will take. This is especially important in either development situates (as a substitution for developer_limit_memory) or when instead of one big leveldb file the application already spreads them around - it also allows to reserve bigger chunks then 20MB if it is helpful for the system.