ALPN is a TLS extension for application-layer protocol negotiation
within the TLS handshake [1].
This patch extends the Pool API to include a new 'alpn_protocols'
parameter. With this parameter, users can set an ALPN preference list
(descending order of preference) to be advertised by load balancer to
members.
This patch also adds HTTP/2 over TLS support to TLS-enabled pools to the
Amphora provider driver, although default the pool ALPN protocol list
configuration setting has HTTP/2 disabled similarly to the default
listener ALPN protocol list value added in Victoria release.
[1] https://tools.ietf.org/html/rfc7301
Change-Id: I91924486bab22601c15c538c8a5282ad8bc54700
d2d5fc80f8