refactor: expand pool and connection implementation

[shollyman]

This PR includes much of the rewiring of the existing ManagedStream abstraction, but doesn't cut over to the new implemention yet.

We add a reference to the origin writer as part of the pendingWrite which retains information about a single write request and response. This allows us to resolve retry settings for a given write by checking if the writer has a custom retry policy. In other cases, we use the default settings of the connection pool.

We introduce internal UUID identifiers to the core abstractions (pool, connection, writer) so that we can add observability later to see which components are responsible for processing requests.

We remove the notion of adding connections to the connectionpool contract. Instead, we introduce a new interface in the pool called a poolRouter. By interface contract, it's responsible for picking the correct connection for a given write. However, this allows us to abstract away different implementations for pool behavior and make it the responsibility of an individual router.

Further, this PR adds the most simplistic router we'll use for the initial migration to multiplexing (simpleRouter): it supports a single connection, and routes all traffic to it.

This PR also moves over more internal functionality from the ManagedStream, namely appendWithRetry() and lockingAppend(). The implementations still remain on the ManagedStream implementation at this time, we'll remove most of the functionality when we cut over to using pools/connections.

Towards: #7103