Add a configurable memory cap for mempool

[congwang]

Currently mempool is unbounded and memory in mempool grows and settles on steady state. This causes stream manager could use a lot of memory in production. We need a mechanism to add a cap there, the memory usage of stmgr is reduced down to 10% and meanwhile performance is almost not affected at all.

Note, I use a global limit for all instances of BaseMemPool (including MemPool), this relies on the C++ type system.

This should resolve #1567

[ajorgensen]

I deployed one of our topologies with this change and I think we're still seeing the stream manager memory usage spike up.

Anything I can do to dig into where its coming from?

[srkukarni]

Is this pr still valid?

[ajorgensen]

I think it was closed because it turned out to not solve the original issue, im not sure if the commits here can still be used but likely that can.

[congwang]

Right, the original back-pressure-related memory issue is addressed by #1785.

About this PR itself, the mempool does not contribute to memory pressure, we will hit the hard limit anyway no matter we have mempool or not, the problem with mempool is that it may hit the soft limit eventually since this memory never shrinks.