Use pool for mutatorContext and transition contexts

Allocating mutatorContext, outgoingTransitionContext and
incomingTransitionContext show up at the top of alloc_space
memory profiling.  Add a typesafe sync.Pool in a new
blueprint/pool package and use it to reuse the context objects.

Test: builds
Change-Id: I4084481fa9b48225fc29538990086d7cf30546bd
4 files changed