exchange: simplify unbundle locking using context managers Differential Revision: https://phab.mercurial-scm.org/D393

util: add base class for transactional context managers We have at least three types with a close() and a release() method where the close() method is supposed to be called on success and the release() method is supposed to be called last, whether successful or not. Two of them (transaction and dirstateguard) already have identical implementations of __enter__ and __exit__. Let's extract a base class for this, so we reuse the code and so the third type (transactionmanager) can also be used as a context manager. Differential Revision: https://phab.mercurial-scm.org/D392

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