[trans-mem] New TM method: serial-irrevocable on first write

[Torvald Riegel <triegel at redhat dot com>]

This adds a new TM method which lets transactions simply read through
without any protection until the first write in the transaction, upon
which the transaction switches to serial-irrevocable mode (thus
isolating all other transactions from uncommitted writes).

This can work well for read-mostly workloads but is too simplistic for
anything else. Nevertheless, it's useful to test the performance of the
upcoming changes to the serial lock implementation, and can be better
than always going to serial mode (which currently is the only
implemented TM method).

OK for branch?

Attachment: patch2
Description: Text document