Restructured interaction between TxManager and KernelAPI

o Unified the way commit/rollback works by means of interaction between
  TxManager and TransactionContext.
o Rest transaction context (TransitionalPeriodTransactionMessContainer)
  uses the TransactionContext instead of TxManager for communicating
  commit/rollback.
o Removed lock releasing checks stemming from confusion about the
  interaction between these two
Merge pull request #755 from tinwelint/2.0-txmgr-txcontext

Restructured interaction between TxManager and KernelAPI