This commit introduces a change in the key format used to reserve/lookup
session-key-indexes. Currently the reservations are stored under the
tower id, however this creates issues when multiple clients are using
the same database since only one reservation is permitted per tower.
We fix this by appending the blob type to the session-key-index locator.
This allows multiple clients to reserve keys for the same tower, but
still limits each client to one outstanding reservation. The changes are
made in a way such that we fall back to the legacy format if the a
reservation under the new format is not found, but only if the blob type
matches blob.TypeAltruistCommit, which is so far the only actively
deployed blob type.
These operations are currently unused, but will be integrated into the
TowerClient at a later point as future preparation for the
WatchtowerClient RPC subserver, which will allow users to add, remove,
and list the watchtowers currntly in use.
This currently takes O(N) time as there does not exist an index of
active client sessions for each watchtower within the client's database.
This index is likely to be added in the future.
This commit replaces the map-based CommittedUpdates field with a slice.
When reading from disk, these will already be sorted by bbolt, so the
client restore the updates as presented without needing to sort them
first.
Since the key in the map variant was the sequence number, we refactor
the CommittedUpdate struct to have a sequence number and an embedded
CommittedUpdateBody (which is equivalent to the old CommittedUpdate).
The database is then expected to populate the sequence number from the
key on disk.
Since the sequence number is now directly integrated in the
CommittedUpdate struct, this allow allows us to remove the now redundant
seqNum argument from CommitUpdate.
