Change messages_data join table indexes for data_id lookup

[peterbroadhurst]

In long runs, we can see this query becomes very inefficient when a large amount of data has built up in the system:

firefly/internal/events/aggregator.go

Lines 609 to 618 in b7937f7

	// Find the messages assocated with that data
	var messages []*fftypes.Message
	for _, data := range data {
	fb := database.MessageQueryFactory.NewFilter(ctx)
	filter := fb.And(fb.Eq("confirmed", nil))
	messages, _, err = ag.database.GetMessagesForData(ctx, data.ID, filter)
	if err != nil {
	return err
	}
	}

[2022-03-31T11:48:27.754Z] DEBUG node_1: SQL-> query: SELECT m.id, m.cid, dbtx=0MOmScfF role=event-manager
[2022-03-31T11:48:28.965Z] DEBUG node_1: SQL<- query dbtx=0MOmScfF role=event-manager

The original intention of this index was it would add a some (unnecessary in my view at this point) additional protection against data ID uniqueness within a message (already enforced strongly in code), while allowing a lookup in either direction to be efficient:

firefly/db/migrations/postgres/000003_create_message_data_join_table.up.sql

Line 9 in b7937f7

CREATE UNIQUE INDEX messages_data_idx ON messages_data(message_id, data_id);

However, the results clearly indicate to me it's only helping with lookup in the message_id based lookups - and not the data_id based lookups. So this PR proposes we replace it with two separate and simple indexes.

[codecov-commenter]

Codecov Report

Merging #653 (829a496) into main (b7937f7) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #653   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          313       313           
  Lines        19175     19175           
=========================================
  Hits         19175     19175           

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