Add acquire timeout support to PostgreSQLMutex #80
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closes #66
Add support for timeouts to PostgresSQLMutex, as mentioned in this issue #66 PostgreSQL supports a non-blocking advisory lock function.
We've needed this a few times and previously I've rolled our own lock/mutex but it'd be easier if it was in the package. Happy to work through any feedback on this one, tried to follow existing patterns, if I missed anything let me know.