Create index on SequencedLeafData(TreeId, LeafIdentityHash)

[robstradling]

In the PostgreSQL storage backend, the selectLeavesByLeafIdentityHashSQL query in log_storage.go will currently do a full table scan on the SequencedLeafData table, because there's no index for joining on its LeafIdentifyHash and TreeId fields. This query is used by the QueueLeaves operation when any of the to-be-queued leaves already exist.

This PR adds an appropriate CREATE INDEX statement in each of the PostgreSQL, MySQL, and CockroachDB storage backends.

Since there's already a corresponding foreign key constraint on the SequencedLeafData table...

FOREIGN KEY(TreeId, LeafIdentityHash) REFERENCES LeafData(TreeId, LeafIdentityHash) ON DELETE CASCADE

...and since MySQL requires that foreign key columns be indexed; if you create a table with a foreign key constraint but no index on a given column, an index is created, it appears that this PR is effectively a no-op for MySQL/MariaDB.

PostgreSQL doesn't automatically create indexes for foreign keys though, so this PR is needed in that context. I'm not sure what the behaviour is for CockroachDB.

I think it makes sense to keep the PostgreSQL, MySQL, and CockroachDB schema scripts as similar as possible, but if it's preferred to reduce the scope of this PR to only update the PostgreSQL backend then I can do that.

Checklist

• I have updated the CHANGELOG.
• I have updated documentation accordingly (including the feature implementation matrix).

[roger2hk]

/gcbrun