rpc: Add experimental config params to allow for subscription buffer size control (tm v0.34.x) (#7230)

A workaround for #6729. Add parameters to control buffer sizes for
event subscription RPC clients. On some networks, buffering causes
clients to be dropped and/or events to be lost.

For additional context, see the discussion on #7188.

- Add experimental_subscription_buffer_size config parameter
- Add experimental_websocket_write_buffer_size config parameter
- Add experimental_close_on_slow_client config parameter

Co-authored-by: M. J. Fromberger <fromberger@interchain.io>