SERVER-15233 allow plan cache commands to run on secondaries is slaveOk explicitly set to true

(cherry picked from commit 77b00970997d13d0758c745e5a94fc79982d4401)

Conflicts:
	src/mongo/s/commands/cluster_plan_cache_cmd.cpp
SERVER-15403 query planner should not error if $min and $max are equal

(cherry picked from commit 88e79e1b476996bab2582009f4ab5ff7c9fad9f4)
SERVER-15696 don't drop filters while creating a SORT_MERGE plan

(cherry picked from commit dcb68b25fe7351888385cc435f339ada8e8b5e9e)
SERVER-14299 prevent mongos from requesting unnecessary batches for sort + limit queries

(cherry picked from commit 8b8a118fc965231293ad56c53a837ffacb17132f)

Conflicts:
	src/mongo/db/query/new_find.cpp
Merge remote-tracking branch 'upstream/v2.6' into v2.6