Connection pooler (#799)

Connection pooler support

Add support for a connection pooler. The idea is to make it generic enough to
be able to switch between different implementations (e.g. pgbouncer or
odyssey). Operator needs to create a deployment with pooler and a service for
it to access.

For connection pool to work properly, a database needs to be prepared by
operator, namely a separate user have to be created with an access to an
installed lookup function (to fetch credential for other users).

This setups is supposed to be used only by robot/application users. Usually a
connection pool implementation is more CPU bounded, so it makes sense to create
several pods for connection pool with more emphasize on cpu resources. At the
moment there are no special affinity or tolerations assigned to bring those
pods closer to the database. For availability purposes minimal number of
connection pool pods is 2, ideally they have to be distributed between
different nodes/AZ, but it's not enforced in the operator itself. Available
configuration supposed to be ergonomic and in the normal case require minimum
changes to a manifest to enable connection pool. To have more control over the
configuration and functionality on the pool side one can customize the
corresponding docker image.

Co-authored-by: Felix Kunde <felix-kunde@gmx.de>

pkg/util/util.go

@@ -147,6 +147,40 @@ func Coalesce(val, defaultVal string) string {
 	return val
 }
 
+// Yeah, golang
+func CoalesceInt32(val, defaultVal *int32) *int32 {
+	if val == nil {
+		return defaultVal
+	}
+	return val
+}
+
+// Test if any of the values is nil
+func testNil(values ...*int32) bool {
+	for _, v := range values {
+		if v == nil {
+			return true
+		}
+	}
+
+	return false
+}
+
+// Return maximum of two integers provided via pointers. If one value is not
+// defined, return the other one. If both are not defined, result is also
+// undefined, caller needs to check for that.
+func MaxInt32(a, b *int32) *int32 {
+	if testNil(a, b) {
+		return nil
+	}
+
+	if *a > *b {
+		return a
+	}
+
+	return b
+}
+
 // IsSmallerQuantity : checks if first resource is of a smaller quantity than the second
 func IsSmallerQuantity(requestStr, limitStr string) (bool, error) {