Add buffer pool for serializing subscription results

[joshua-spacetime]

That is, specifically for subscribe and unsubscribe. Currently for a subscribe(unsubscribe) call, the construction of each TableUpdate/BsatnRowList incurs a heap allocation which is then discarded by the task that transmits results to clients. We should grab buffers from a pool and use those buffers in TableUpdates instead of constantly allocating and deallocating new ones.

[joshua-spacetime]

Part of what is described in this ticket is to avoid allocating an intermediate vec before serializing to a BsatnRowList. This has been implemented separately by #2918.