At the suggestion of Willy Tarreau, the scope of the 'ot.uuid' variable was
changed from 'sess' to 'txn', so it is now limited to the transaction only.
This one is the last optional module to build with haproxy, so let's move
it to addons/. It was renamed to "ot" as it was the only one whose USE_*
option did not match the directory name, now this is consistent.
Few changes were required, only the Makefile, and doc were adjusted, as
the directory was already self-contained and relocatable.