SERVER-6956 Chunk::splitIfShould does not acquire the distributed balance lock when moving chunks

Convert verify into an errmsg to make it more friendly with the debug build test suite. Also fixed
MigrateStatusHolder so it will not accidentally cleanup migrateFromStatus logically owned by another
thread.
SERVER-6634 Handle negative and invalid skip values in count command

Conflicts:
	src/mongo/db/instance.cpp
SERVER-7108 Better message for uassert 9517
SERVER-2771 preliminary code cleanup; no functional change

Conflicts:
	src/mongo/SConscript
	src/mongo/db/btree.cpp
	src/mongo/db/btreecursor.cpp
	src/mongo/db/client.cpp
	src/mongo/db/clientcursor.cpp
	src/mongo/db/cloner.cpp
	src/mongo/db/commands/distinct.cpp
	src/mongo/db/commands/mr.cpp
	src/mongo/db/compact.cpp
	src/mongo/db/curop.cpp
	src/mongo/db/curop.h
	src/mongo/db/cursor.cpp
	src/mongo/db/dbcommands.cpp
	src/mongo/db/dur_recover.cpp
	src/mongo/db/dur_recover.h
	src/mongo/db/extsort.cpp
	src/mongo/db/index_update.cpp
	src/mongo/db/instance.cpp
	src/mongo/db/pdfile.cpp
	src/mongo/s/d_migrate.cpp
	src/mongo/util/touch_pages.cpp
buildbot sharding_balance4.js explicitly report chunk manager sequence number when available in setShardVersion
Introduced constants to refer to collection names and fields in the config server.

Conflicts:
	src/mongo/SConscript
	src/mongo/s/commands_admin.cpp
	src/mongo/s/config.cpp
	src/mongo/s/config_migrate.cpp
	src/mongo/s/shard.cpp
Reorder query criteria for chunks query to make more efficient.