Allow to use channel manager provided via config

[beargummy]

Sometimes it could be useful to provide custom ChannelManager and/or reuse it, like it is done with Timer.
Also, to support this feature, method releaseChannelLock is being reused in DefaultChannelGroup#remove, since logic is effectively the same.

[slandelle]

Sorry, but that's not possible. ChannelManager is an internal, not a public API, so it can can any time. Please explain your actual use case.

[beargummy]

@slandelle actual use case is to be able to use custom channel(connections) limitations, not only global and per-host. This is partially achievable via custom ChannelPoolPartitioning with necessary logic, but it lacks custom limits. E.g. I want to limit per-host connections to some huge number of connections and also add different (lower) custom limit based on some request parameters.

[slandelle]

Looks like you need something related to ConnectionSemaphore. Feel free to suggest API changes for your use case.

[beargummy]

I'd like to, and I definitely will, but ConnectionSemaphore is part of 2.4.x version, and unfortunately I cannot migrate to this version due to open netty's issue. That's why I want to provide custom ChannelManager.

[slandelle]

IIRC, issue #6706 has actually always been there and is not related to a Netty upgrade.