Add support for the TLS padding extension.

This works around a bug in some versions of F5 devices that causes
the connection to hang when the ClientHello record is between 256 and
511 bytes long.

https://tools.ietf.org/html/draft-agl-tls-padding-02

Based on the upstream patch to do the same thing:
0467ea68624450ecece4cde0d5803499aaff19c2

Change-Id: I67b3221706b9257b0b2ff45d2d418d37a44abf8c