Add markov_all_index to the database

This speeds up markov all quite a bit.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

db.sql

@@ -28,3 +28,5 @@ CREATE TABLE IF NOT EXISTS chain (
 );
 
 CREATE UNIQUE INDEX IF NOT EXISTS chain_value_next ON chain(user, value, next);
+
+CREATE INDEX IF NOT EXISTS markov_all_index ON chain(value);