markov: Speed up !markov all, h/t charlz

Add a new index to the chain.value column

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

data/markov/db.sql

@@ -17,3 +17,5 @@ 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);