Mv flexcache

This is the eleveldb side of mv-flexcache branch (already reviewed) in basho/leveldb.  The branch creates 3 config atoms:  total_memory, total_leveldb_mem, and total_leveldb_mem_percent.  Engel wrote code for total_memory.  The value is populated upon each database open for potential use with total_leveldb_mem_percent.  Any configuration value supplied by the user for total_leveldb_mem or total_leveldb_mem_percent is placed in the leveldb::Options.total_leveldb_mem structure member.  If between 0 and 100, it is assumed later to be a percent and applied against total_memory to achieve a byte value.