fatxpool: improve event listeners

[michalkucharczyk]

Some improvements can be made to internal eventing system:

• update_view: InBlock event could be sent via side-channel, instead of registering listener in view (in case transaction is already in the cloned view). This can simplify the code.
• TxInMemPool::watched - this could be replaced with listener, TxMemPool::listener would become redundant.
• optimization: view should communicate with MVL over the aggregated stream instead of per-transaction streams. This should greatly increase performance (e.g. registering per-tx listeners in new view is very costly now).

[michalkucharczyk]

update_view: InBlock event could be sent via side-channel, instead of registering listener in view (in case transaction is already in the cloned view). This can simplify the code.

As long as we have old pool, we need to send events via listener. So no big gain here. Also aggregated stream reduces the overall cost of eventing (at least in maintain thread).

TxInMemPool::watched - this could be replaced with listener, TxMemPool::listener would become redundant.

I reviewed the code, and I don't consider this point to be valid. I think what we have today is well structed, moving listener will not improve existing codebase.

optimization: view should communicate with MVL over the aggregated stream instead of per-transaction streams. This should greatly increase performance (e.g. registering per-tx listeners in new view is very costly now).

This was valid point, and is addressed in dedicated issue: #7071