Prevent crashes because of too many opened file descriptors

We fixed crashed because of too many opened file descriptors. The issue was not visible quickly but when Drive was running a long time with a lot of synchronization actions. Each thread was opening multiple connections to the SQLite database, but they were never released. Resulting in an increase of the number of opened file descriptors and it was consuming more and more memory (RAM). Both situations were ending with a hard crash: the OS was killing the application in both cases.