implement LISTCORRESPONDENTS

[slingamn]

From discussion here:

ircv3/ircv3-specifications#444

[slingamn]

Note: if we end up removing the * target, we'll no longer need to include nickname targets in the sequence table, with the consequence that entries in sequence will be uniquely identified by sequence.history_id. It will then be possible to reconfigure sequence such that history_id is its primary key: alter table sequence drop column id, add primary key(history_id);.

[slingamn]

Or rather, we won't automatically execute that alter (it will typically fail due to a duplicate key error), but we can change the relevant CREATE TABLE command for new installs without breaking anything, since we never actually query sequence.id.

[slingamn]

This is a little tricky because we currently assume that the timestamp is available in sequences. We could join against both sequences and conversations to get it...

[slingamn]

The current spec for this is ircv3/ircv3-specifications#450