Add requestkey indexes on events and transactions

As identified by @blazesal in
#79, our `/txs/tx`
endpoint requires a requestkey index on the events and transactions
tables, otherwise each request results in a table scan.

I've just tried creating these indexes manually and checking their
sizes; As of now, the requestkey index on the transactions table takes
up 603MB of space while the one on events takes up 879MB. For
comparison, these total space occupied by these two tables is ~46GB.

exec/Main.hs

@@ -69,6 +69,8 @@ main = do
                 addEventsHeightNameParamsIndex logg conn
                 addFromAccountsIndex logg conn
                 addToAccountsIndex logg conn
+                addTransactionsRequestKeyIndex logg conn
+                addEventsRequestKeyIndex logg conn
             logg Info "DB Tables Initialized"
             let mgrSettings = mkManagerSettings (TLSSettingsSimple True False False) Nothing
             m <- newManager mgrSettings
@@ -170,6 +172,24 @@ addToAccountsIndex =
     , statement = "CREATE INDEX IF NOT EXISTS transfers_to_acct_height_idx_idx ON transfers (to_acct, height desc, idx);"
     }
 
+addTransactionsRequestKeyIndex :: LogFunctionIO Text -> Connection -> IO ()
+addTransactionsRequestKeyIndex =
+  addIndex
+    IndexCreationInfo
+    {
+      message = "Adding \"(requestkey)\" index on transactions table"
+    , statement = "CREATE INDEX IF NOT EXISTS transactions_requestkey_idx ON transactions (requestkey);"
+    }
+
+addEventsRequestKeyIndex :: LogFunctionIO Text -> Connection -> IO ()
+addEventsRequestKeyIndex =
+  addIndex
+    IndexCreationInfo
+    {
+      message = "Adding \"(requestkey)\" index on events table"
+    , statement = "CREATE INDEX IF NOT EXISTS events_requestkey_idx ON events (requestkey);"
+    }
+
 {-
 λ> :main single --chain 2 --height 1487570 --service-host api.chainweb.com --p2p-host us-e3.chainweb.com --dbname chainweb-data --service-port 443 --service-https
 λ> :main single --chain 0 --height 1494311 --service-host api.chainweb.com --p2p-host us-e3.chainweb.com --dbname chainweb-data --service-port 443 --service-https