Introduce TCP & UDP server port scanning utility

Prior to this commit the Spring Framework did not provide a public means
for scanning for available server ports. However, the Spring Framework
internally used a FreePortScanner in integration tests within its own
test suite. Furthermore, Spring Integration 2.2 provides similar support
in a SocketUtils class in the spring-integration-test module.

This commit introduces SocketUtils in spring-core to replace the
FreePortScanner which was previously only used internally within
Spring's test suite. This new implementation is inspired by both Spring
Framework's FreePortScanner and Spring Integration's SocketUtils and
consequently attempts to merge the best of both previous
implementations.

Issue: SPR-8032
Merge from sbrannen/SPR-8032

* SPR-8032:
  Introduce TCP & UDP server port scanning utility