Add helper method for preparing lists of statements. (#25)

* Add helper method for preparing lists of statements.

src/github.com/matrix-org/dendrite/roomserver/storage/prepare.go

@@ -0,0 +1,21 @@
+package storage
+
+import (
+	"database/sql"
+)
+
+// a statementList is a list of SQL statements to prepare and a pointer to where to store the resulting prepared statement.
+type statementList []struct {
+	statement **sql.Stmt
+	sql       string
+}
+
+// prepare the SQL for each statement in the list and assign the result to the prepared statement.
+func (s statementList) prepare(db *sql.DB) (err error) {
+	for _, statement := range s {
+		if *statement.statement, err = db.Prepare(statement.sql); err != nil {
+			return
+		}
+	}
+	return
+}