perf: Prune unused pattern branches

[benclive]

What this PR does / why we need it:
Adds a Prune command to Drain to clear branches with no associated log clusters.

• We use an LRU cache to limit the number of patterns we match against
• But when we delete a pattern due to age, we don't delete it's corresponding branches in the drain tree.
• This PR adds a step to traverse the tree and delete any nodes that don't have children or log clusters attached.
• It is run periodically, at the same time as we run the current logcluster prune process but it could probably be reduced in frequency.

[cyriltovena]

LGTM

[benclive]

After deploying in ops,
memory has improved (less frequently crashes) though it still needs work :

CPU hasn't noticably changed:

Gonna merge this so we see this improvement everywhere.