…state (#867)

When OperationTimedOutException fires in SpeculativeExecution.onTimeout(),
log the configured driver-side timeout, per-connection in-flight count,
pool pending borrow queue length, and total pool in-flight count.

This helps distinguish between two failure modes:
- Pool contention (requests queued inside the driver before reaching server):
  visible as pool pending borrows > 0
- Server-side slowness (requests reached server but driver gave up first):
  visible as pool pending borrows = 0 with high connection in-flight

To expose the timeout value, ResponseHandler.readTimeoutMillis is widened
from private to package-private. All access remains within
com.datastax.driver.core.

Motivated by CUSTOMER-301 (Whoop): client-side OperationTimedOutException
bursts with no corresponding server-side timeouts, requiring diagnostics
to determine whether the bottleneck is in the driver pool or on the server.