new schema: add index to optimise incoming message check

On a fairly busy setup with many thousand messages in the database, I was finding that the check for incoming messages (NativeConnectionHandler.checkMessages(), run every 250ms) was causing PostgreSQL to use a lot of CPU time.  The query for incoming messages required a sequential scan of the ttirc_messages table.

Adding an index on the 'incoming' column of this table makes this query much faster, since most of the messages will have incoming=true and PostgreSQL can immediately ignore those.

I've created a new schema version (10) with this index and tested that schema updates work as expected.

Many thanks for an extremely useful app!