Work around the JVM crash that occurs when cipher suite uses GCM

Motivation:

For an unknown reason, JVM of JDK8 crashes intermittently when
SslHandler feeds a direct buffer to SSLEngine.unwrap() *and* the current
cipher suite has GCM (Galois/Counter Mode) enabled.

Modifications:

Convert the inbound network buffer to a heap buffer when the current
cipher suite is using GCM.

Result:

JVM does not crash anymore.