Turn of gc (`rc.gc=0`) for read operations (at least until TW does this)

With TW3, each time a task command is run, task runs garbage collection---this modifies the database (and it's modification time). This has a few unwanted effects:

- the run is slower because of garbage collection
- the database is modified, which can cause issues with syncing software (for those of us using things like syncthing that rely on file modification times).

So, for "read" operations here in tasklib, perhaps garbage collection should be turned off by default? I see that this is done in one location where we "refresh" tasks, but maybe it can be done everywhere where we're not using a "write" operation?

https://github.com/GothenburgBitFactory/tasklib/blob/793a86d2432d93425e36a6384db1f563be07018c/tasklib/backends.py#L387

Some of this is also being discussed in TW3 related to performance etc. E.g.:

https://github.com/GothenburgBitFactory/taskwarrior/issues/3418