Replace "synchronized" blocks in AbstractSession with ReentrantLock

[noxvost]

Synchronized blocks pin the virtual thread under JDK21+ to physical thread. It is better to avoid such pinning in I/O bound operations so VirtualThread planner can do its job

[tomaswolf]

There are a lot more synchronized blocks and methods in Apache MINA sshd. I'm fine with replacing these two; I would object to replacing all uses of synchronized by re-entrant locks.

If thread pinning is a problem, I think you could migrate to Java >= 24, which should have JEP-491 implemented: using synchronized together with virtual threads should not be a problem anymore.