Accessing the repository's config and immediately taking a snapshot of
it is a common operation, so let's provide a convenience function for
it.

Now that our strmap is no longer modified but replaced, we can use the
same strmap for the snapshot's values and it will be freed when we don't
need it anymore.

This is mostly groundwork to let us re-use the map in the snapshots.

When we delete an entry, we also want to refresh the configuration to
catch any changes that happened externally.

This allows us to simplify the logic, as we no longer need to delete
these variables internally. The whole state will be refreshed and the
deleted entries won't be there.

With the isolation of complex reads, we can now try to refresh the
on-disk file before reading a value from it.

This changes the semantics a bit, as before we could be sure that a
string we got from the configuration was valid until we wrote or
refreshed. This is no longer the case, as a read can also invalidate the
pointer.

When writing out, parse the resulting file instead of adding or
replacing the value locally. This has the effect of reading external
changes as well.

This will be used by the writing commands in a later step.

Current code sets the active map to a new one and builds it whilst it's
active. This is a race condition with someone else trying to access the
same config.

Instead, let's build up our new map and swap the active and new one.