Extract tcb & lastUsed to its own lock
Extract tcb & lastUsed to its own lock

These fields do not need to synchronize reads/writes with the rest of
the connection.

PiperOrigin-RevId: 407183693

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor