Simplify channel management
Simplify channel management

The channels {cancel,resCh} have roughly the same lifetime and are used for
roughly the same purpose as an entry's waiters; we can unify the state
management of the two mechanisms, while also reducing unncessary mutex locking
and unlocking.

Made some cosmetic changes while I'm here.

PiperOrigin-RevId: 218343915
Change-Id: Ic69546a2b7b390162b2231f07f335dd6199472d7

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