When migrating the actions store from kvdb to sql, we will update the
existing actions to include the full mac root key, instead of just the
last 4 bytes (currently called `MacaroonIdentifier`). In order to do so,
we change the sql implementation of the `actions` store to persist the
full mac root key, instead of just the last 4 bytes. As no production
data in the sql actions store exists for users yet, it's fine for us to
change this without having to address old sql actions which only stored
the last 4 bytes.

Note though that we do not update the kvdb implementation, and the full
macaroon root key will be ignored by the kvdb store even if set.
Therefore, the rest of `litd` will still have to just expect the last 4
bytes of the mac root key when accessing an `Action`'s
MacaroonIdentifier. Therefore, we we currently never expose the rest of
the mac root key outside of the sql actions store.

Once the kvdb store has been fully deprecated and removed, we can then
update the rest of `litd` to also use the full mac root key, and change
the `Action` struct's field to reflect this.

Add a new SQL query `GetAction` to retrieve a single action by its ID.
This query will be needed for the kvdb to SQL migration of actions
store.

In the upcoming kvdb to SQL migration of the actions store, we need to
simulate in tests that two or more accounts have colliding account
aliases for the first 4 bytes of the alias. In order to allow creation
of such accounts, we need to be able to update the alias of an account
in tests, and this commit adds the a SQL query enabling this
functionality.

Note that the `UpdateAccountAliasForTests` query is only intended for
use in tests and should not be used in production code.

In preparation for the kvdb to SQL migration of the actions store, this
commit adds an `actions` field to the expected result of the migration
tests. Once the migration is implemented, this field will be used to
validate that the migrated actions match the expected results.

This commit adds an `accountStore` and a `rootKeyStore` arg the database
population functions of the kvdb to SQL migration tests of the
firewalldb.

As an action can be linked to an account, we need to enable simulation
of that in the migration tests of the actions store. In order to create
the accounts to link the actions to, we need to create the accounts in
the account store, which therefore requires passing the `accountStore`
to database population functions of the migration tests.

As the kvdb to SQL migration also will update the migrated actions to
not only store the 4 byte short ID of the action's corresponding
macaroon, but to it's full 8 byte root key ID. This requires the
migration function has access to all of lnd's 8 byte root key IDs, and
the migration function will therefore be change to accept a [][]byte
arg containing all of lnd's root key IDs.
As we can't access a full lnd instance in the migration unit tests, we
need to create a mock instance that simulates the root key store, and
this commit therefore adds mock `rootKeyStore` struct which is also
passed to the database population functions of the migration tests.
This `rootKeyStore` struct can be used to generate dummy root key IDs
when creating simulated actions in the migration tests.

This commit introduces the migration logic for transitioning the
actions store from kvdb to SQL.

Note that as of this commit, the migration is not yet triggered by any
production code, i.e. only tests execute the migration logic.