test: add concurrent load testing for channel opening

[Arshia-r-m]

The following test simulates a scenario where a single node functions as the maker, and multiple takers request to open a channel to it. In this case, the node panics!

[zoedberg]

Thanks for reporting this! It seems there's a bug in the handling of the lock that should protect from multiple open channel requests. I've fixed it, will push it soon, but please first rebase this on top of the latest master tip and rename the test and its file to concurrent_openchannel (please remember also to update the test directory accordingly).

[zoedberg]

Pushed the fix on top of your commit, merging this.
As you can see the bug was related to an incorrect usage of the unlocked state mutex that exists to protect concurrent calls to the APIs. This was preventing the rgb_send_lock to work properly (to prevent double spends).
To make the test pass I also added a test utility to retry the openchannel request in case it receives a forbidden error.
Finally I also improved a bit the handle event code, where we can now leverage the ReplayEvent error to allow retrying events when an error occurs.