Database Lock Prevention

[groue]

This pull request makes it possible to prevent acquisition of locks on a database.

It aims at providing the necessary ground-level work for the prevention of 0xdead10cc exception in iOS apps that store their database in the container on an App Group.

https://developer.apple.com/library/archive/technotes/tn2151/_index.html:

The exception code 0xdead10cc indicates that an application has been terminated by the OS because it held on to a file lock or sqlite database lock during suspension. If your application is performing operations on a locked file or sqlite database at suspension time, it must request additional background execution time to complete those operations and relinquish the lock before suspending.

We can thus prevent the 0xdead10cc exception if we can release and prevent locks between the notification that the additional background execution time is about to expire, and the end of the suspended state.

In this pull request, we only address the exclusive lock prevention itself, not its scheduling.

dbQueue.startPreventingLock() // or dbPool
dbQueue.stopPreventingLock()  // or dbPool

Those methods are public because they are defined on the DatabaseReader protocol. But they are not supposed to be used by the GRDB user. They won't be documented in the main README. They will be used by a future pull request which will target 0xdead10cc specifically.

Between startPreventingLock() and stopPreventingLock(), all database accesses may throw a DatabaseError of code SQLITE_INTERRUPT, or SQLITE_ABORT, except reads in WAL mode.

---

Update

The followup pull request #668 has changed the vocabulary used in this PR. We know talk about "suspended" databases, and the start/stop methods have been renamed, and made internal.