Move the pendingTransaction stuff into BLASTSyncEngine

The functionality has morphed since its original transitions-only
incarnation. Now it is a general queue mechanism for SyncEngine.

So, move it into BLASTSyncEngine and clean-up the abstraction
a little-bit. This should make it more usable for some upcoming
fixes.

Bug: 222033492
Test: existing tests pass (just a refactor)
Change-Id: Ia057e816ad5635e9681d3273d139e494c18216d9