database/sql: Avoid re-preparing statements when all connections are busy

Previously, if all connections were busy, we would always
re-prepare the statement on the connection we were assigned from
the pool.  That meant that if all connections were busy most of the
time, the number of prepared statements for each connection would
keep increasing over time.

Instead, after getting a free connection, check to see if the
statement has already been prepared on it, and reuse the statement
handle if so.

LGTM=bradfitz
R=golang-codereviews, gobot, bradfitz
CC=golang-codereviews
https://golang.org/cl/116930043

src/pkg/database/sql/sql_test.go

@@ -1348,6 +1348,11 @@ func TestErrBadConnReconnect(t *testing.T) {
 		return nil
 	})
 
+	// Provide a way to force a re-prepare of a statement on next execution
+	forcePrepare := func(stmt *Stmt) {
+		stmt.css = nil
+	}
+
 	// stmt.Exec
 	stmt1, err := db.Prepare("INSERT|t1|name=?,age=?,dead=?")
 	if err != nil {
@@ -1355,9 +1360,7 @@ func TestErrBadConnReconnect(t *testing.T) {
 	}
 	defer stmt1.Close()
 	// make sure we must prepare the stmt first
-	for _, cs := range stmt1.css {
-		cs.dc.inUse = true
-	}
+	forcePrepare(stmt1)
 
 	stmtExec := func() error {
 		_, err := stmt1.Exec("Gopher", 3, false)
@@ -1373,9 +1376,7 @@ func TestErrBadConnReconnect(t *testing.T) {
 	}
 	defer stmt2.Close()
 	// make sure we must prepare the stmt first
-	for _, cs := range stmt2.css {
-		cs.dc.inUse = true
-	}
+	forcePrepare(stmt2)
 
 	stmtQuery := func() error {
 		rows, err := stmt2.Query()