Adjust to new redlock behaviour

[nduitz]

There have been some changes to the behaviour of redis-client and redlock-rb which lead to a different behaviour in this gem.

Before leandromoreira/redlock-rb#110 was merged, redlock would return false if there was an error with the connection, now it raises the error.
Before this behaviour could have been handled by configuring on_conflict. Now the behaviour should be more fine grained.

This adds a on_error option, which can be either nil (leading to a reraise) or a proc leading to the evaluation of said proc.

This allows users to handle connections errors to reduce the amount of jobs that get lost because of said connection errors.

[sharshenov]

Hey @nduitz
Thank you for the PR. I'm happy to accept it. But at the same time it would be nice to preserve compatibility with older versions of Ruby.

[sharshenov]

This syntax is not available in Rubies pre-3.1

[nduitz]

thanks for the fast feedback :) will change.

You've been fast, in the meantime i've changed the naming a bit and added support for a global config

[nduitz]

I've been running this PR in production for some days now.
I added some logging around that callback, but it appears to be silent. So this might not be an issue at all. Even though this PR might still be a feature worth adding.

Most of the errors that we encounter come from delete_lock. So I will try and investigate that if I find the time, maybe there is something we can do about that.
redlock's strategy is to simply fire and forget the unlock attempt, rescuing every error that might occur. But I don't know about that, since redlock is more about process based locking. Trying the same might lead to a deadlock situation.