TransactionManager getId and getContext

TransactionParticipants get called by the TransactionManager using
their prepare, prepareForAbort, commit and abort callbacks, but for
situations where one needs access to the Context in a deeper class,
called by the participant (i.e. Managers), we now have a couple of
static ThreadLocal based methods:

• Serializable getSerializable()
• Context getContext() (in case your Serializable is actually an instance of
  org.jpos.transaction.Context)
• Long getId ()

Please note returned values may be null when run outside the TM life-cycle.

Also note that the TM takes care of PAUSED transactions, setting
these values on the resumed thread.

See ChangeLog – 6da5f3 for details.