tsan: optimize vector clock operations

Make vector clock operations O(1) for several important classes of use cases.
See comments for details.
Below are stats from a large server app, 77% of all clock operations are handled as O(1).

Clock acquire                     :         25983645
  empty clock                     :          6288080
  fast from release-store         :         14917504
  contains my tid                 :          4515743
  repeated (fast)                 :          2141428
  full (slow)                     :          2636633
  acquired something              :          1426863
Clock release                     :          2544216
  resize                          :             6241
  fast1                           :           197693
  fast2                           :          1016293
  fast3                           :             2007
  full (slow)                     :          1797488
  was acquired                    :           709227
  clear tail                      :                1
  last overflow                   :                0
Clock release store               :          3446946
  resize                          :           200516
  fast                            :           469265
  slow                            :          2977681
  clear tail                      :                0
Clock acquire-release             :           820028

llvm-svn: 204656