3.x: fix keyspace setup failures escaping request retry path

[dkropachev]

Parent issue: #939

Scope

Fix the retry bypass where keyspace setup can fail synchronously on a defunct or otherwise unusable connection before RequestHandler installs its normal borrow callback.

The main observed path is:

HostConnectionPool.borrowConnection()
  -> Connection.setKeyspaceAsync(...)
  -> Connection.write(USE "<keyspace>")
  -> ConnectionException: Write attempt on defunct connection

Because the exception is thrown before borrowConnection() returns a ListenableFuture, the request is wrapped as DriverInternalError instead of being retried on the next host.

Proposed Changes

• Make Connection.setKeyspaceAsync() convert synchronous ConnectionException / BusyConnectionException from the internal USE write into a failed future.
• Reset targetKeyspace when keyspace setup fails synchronously so future attempts are not blocked by a stale in-flight attempt.
• Ensure HostConnectionPool.borrowConnection() restores inFlight / totalInFlight accounting when keyspace setup fails before the borrower receives a connection.
• Ensure HostConnectionPool.dequeue() handles keyspace setup failures for pending borrows by completing the pending borrow with the failure and restoring connection counters.
• Consider skipping or re-checking defunct connections before keyspace setup after inFlight is incremented.

Acceptance Criteria

• A defunct connection selected during borrowConnection() keyspace setup returns a failed future instead of throwing synchronously.
• RequestHandler reaches the existing onFailure() path and can try the next host when available.
• Pending borrows fail cleanly if keyspace setup fails during dequeue.
• Pool counters are restored on failed keyspace setup; no inFlight / totalInFlight leak.
• Tests cover direct borrow and pending-borrow dequeue keyspace setup failures.