refs #399, have a background thread run a hot_optimize
on the oplog once every 5 seconds. The hot optimize only
goes until the beginning of non-deleted data.
Merge branch 'bugs/399'

Conflicts:
	src/mongo/db/index.cpp
	src/mongo/db/index.h
	src/mongo/db/namespace_details.h
	src/mongo/db/repl/rs.cpp