[trans-mem] Release transaction objects on thread termination

[Torvald Riegel <triegel at redhat dot com>]

This patch removes the per-thread free-list of gtm_transaction objects.
With a single stable gtm_transaction object per thread, it is not
necessary anymore. Furthermore, a destructor function of a pthread TLS
key is used to release a thread's gtm_transaction object when this
thread terminates. This works even when transactions are used in other
TLS key's destructors because the key is initialized whenever a
transaction is used for the first time in a thread (or after a previous
destruction in this thread).
The test for the release doesn't really check that the release happens
(and can't because we do black-box testing), but at least it will
trigger the release twice.

OK for branch?

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