FDCache: implement a basic sharding of the FDCache

This is just a basic sharding. A more sophisticated implementation would
rely on something other than luck for keeping the distribution equitable.

Signed-off-by: Greg Farnum <greg@inktank.com>

Conflicts:
	src/common/config_opts.h
	src/os/FDCache.h

Resolved the conflicts

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>