Fix race condition in next_block_for
Fix race condition in next_block_for

If we hit the timeout while not blocked on read, the timeout would
overflow.

sfackler/rust-postgres,sfackler/rust-postgres,achanda/rust-postgres,achanda/rust-postgres,ddrmanxbxfr/rust-postgres,ddrmanxbxfr/rust-postgres,tempbottle/rust-postgres,tempbottle/rust-postgres