This change changes the registries accesses to use GetTransaction which
determines which kind of access it's dealing with (e.g,. snap set,
snapctl set, in or out of a hook, registry hook or non-registry, etc)
and takes the appropriate action.
This omits the concurrency checks and I've also opted to removed the
bits allowing reading concurrently if the registry being read is
different than the one with a transaction ongoing (this will require
changes anyway since we'll add a load-registry change in the future).
At the moment, the user API endpoint isn't connected to this yet since
that will require more changes to it.

Signed-off-by: Miguel Pires <miguel.pires@canonical.com>

Signed-off-by: Miguel Pires <miguel.pires@canonical.com>

Signed-off-by: Miguel Pires <miguel.pires@canonical.com>

Make reads through snapctl go through a different flow than the writes.
This will change in the future as the reads will also trigger a change
to load data but for now this makes access logic easier to reason about
and it's also slightly more correct.

Signed-off-by: Miguel Pires <miguel.pires@canonical.com>

Signed-off-by: Miguel Pires <miguel.pires@canonical.com>

Signed-off-by: Miguel Pires <miguel.pires@canonical.com>