refactor channel refresh to avoid internal api_core references

[daniel-sanche]

The client's _manage_channel logic currently updates internal _grpc_channel references to refresh the grpc channel. This can cause issues with future gapic versions, because it's not part of the advertised, stable API.

We should instead move this logic into a grpc.Channel wrapper, and pass the wrapped channel in when creating the transport. That should allow us to keep our customizations in a more stable way, while also pushing complexity outside the BigtableClient class