exchange: remove need for "locked" variable
The transactionmanager() constructor just assigned a few variables and
cannot fail, so it's safe to move it inside the earlier try/except.
Differential Revision: https://phab.mercurial-scm.org/D391
exchange: drop now-unnecessary "local" from lock name variables
Since
fda0867cfe03 (exchange: drop support for lock-based unbundling
(BC), 2017-08-06), there is no more remote locking.
Differential Revision: https://phab.mercurial-scm.org/D390