[DRAFT] feat: dynamic channel pool

[daniel-sanche]

This PR adds dynamic channel pooling, where channel pools will resize depending on channel usage patterns. To implement this, I moved more logic into grpc channel subclasses, resulting in the following classes:

• _WrappedChannel: a generic class that holds a grpc channel, and delegates all calls to the wrapped object
• _BackgroundTaskMixin: an interface/mixin class that provides logic for starting and managing a background task associated with a grpc channel
• RefreshableChannel: a channel that uses _BackgroundTaskMixin and _WrappedChannel to periodically refresh the wrapped channel, and call a warm up callback when the new channel is set up. This replaces some logic that was currently in the client
• TrackedChannel: a _WrappedChannel that keeps tracks of the counts number of active rpcs as they are sent
• PooledChannel: a grpc channel subclass that maintains a static pool of channels, and alternates incoming requests between them. For our use-case, (if we allow static channels) each channel in the pool will be a RefreshableChannel.
• DynamicPooledChannel: a PooledChannel that wraps each channel in the pool in a TrackedChannel, and maintains a _BackgroundTaskMixin task to periodically change the pool size based on channel usage.

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TODO:

• check performance, make optimizations as needed
  • possible optimizations:
    • create a class that combined RefreshableChannel with TrackedChannel, to cut down on the layers of wrapping
    • create CacheableChannel that saves and returns a cahed MultiCallable, to avoid the overhead of creating one on each call (does grpc already do this internally?)
• add system tests
• get full unit test coverage

Later:

• move most classes in _channel_pooling into api_core, to share with other libraries