SERVER-1752 Improve performance of simple counts by avoiding use of a matcher when an optimal btree cursor can filter results internally.

Conflicts:
	src/mongo/db/namespace_details.h
	src/mongo/db/ops/count.cpp
	src/mongo/db/ops/query.cpp
	src/mongo/db/queryoptimizercursorimpl.cpp
	src/mongo/dbtests/cursortests.cpp
	src/mongo/dbtests/queryoptimizercursortests.cpp
SERVER-1752 Remove unused Cursor::matcherPtr() function.
SERVER-1752 SERVER-4529 Do not incorrectly use fast count mode when matching against a null value.
SERVER-1752 Within BtreeCursor::currentMatches(), filter keys that do not match the FieldRangeVector's index bounds.
Merge branch 'master' into fastcount
Fix