* Move ConnectionCache back to solana-client, and duplicate ThinClient, TpuClient there
* Dedupe thin_client modules
* Dedupe tpu_client modules
* Move TpuClient to TpuConnectionCache
* Move ThinClient to TpuConnectionCache
* Move TpuConnection and quic/udp trait implementations back to solana-client
* Remove enum_dispatch from solana-tpu-client
* Move udp-client to its own crate
* Move quic-client to its own crate
* Add new empty crates and tpu_connection_cache module
* Add BaseTpuConnection trait and impl for udp and quic
* Add ConnectionPool trait and impl for udp and quic (quic-client doesn't build)
* Add a couple quic-specific apis to QuicConfig (from ConnectionCache)
* Re-export quic- and udp-client modules to prep for move
* Move ConnectionCacheStats to new module
* Move consts
* Duplicate ConnectionCache into tpu_connection_cache (doesn't build)
* Move methods to QuicConfig and remove unneeded methods (doesn't build)
* Genericize new ConnectionCache
* Rename new struct
* Copy unit tests (tests don't build)
* Fixup unit tests
* Move quic-specific test to quic-client crate
* DRY by using ConnectionPool method
* Prevent evicting pool about to be used