OTS11 — OTS-RTF issue: Synchronization very (too) expensive

The Synchronization mechanism defined by the Transaction Service
is expensive (in terms of performance). It adds three ORB-
mediated (typically remote) calls per transaction:

• the registration call
• the before_completion call
• the after_completion call

This is far from optimal for applications that register the same
same Synchronization for all their transactions (would be nice
if a single registration could be sufficient), for applications
that don't need the before_completion callback (why pay for this
extra no-op call?), and for applications that don't need the
after_completion callback (why pay for this extra no-op call?).