Add connection pooling to exp module

[aisk]

Make the exp clients cloneable and shareable by pooling connections per server.

• Each server keeps a stack of idle connections behind a mutex: checkout pops one or dials a new connection (never blocks), return keeps it only while under with_max_idle (default 8); there is no cap on concurrent connections, only on retained idle ones, so batches can never deadlock on checkout
• A connection that fails mid-exchange (io error, EOF, framing error) is dropped instead of reused, since the stream state is unknown; errors after a completely framed response keep the connection
• Both clients are now Clone + Send + Sync with all methods on &self; clones share the pools; from_connection is removed since a pooled client needs a dialable address
• Addresses are resolved eagerly at connect time and one connection per server is dialed up front, so connection errors still surface at connect
• The duplicated batch prepare/grouping logic moves into a shared plan() in the core