IssueTracker

Component used: Room (+RxJava support)
Version used: 2.0.0
Devices/Android versions reproduced on: Any

We recently started seeing a lot of lock contention upon subscription to Observables provided by Room. At first we thought that the problem was caused by `syncTriggers` doing DB work on the main thread (a known issue here: https://issuetracker.google.com/issues/117201279), but it appears that acquiring the close lock is causing the most slowdown.

It seems that the close lock is unintentionally being used to obtain an exclusive lock on the entire database, blocking any other subscriptions while there is either another subscription happening at the same time or the refresh runnable is running. I'm not sure why the protection against closing is required in the first place, but maybe a `ReentrantReadWriteLock` could accomplish the intended goal without the exclusiveness.