TS-2802: add SNI support for origin server connections

Set the SNI name when making SSL origin server connections. Update
IP address literal parsing to accept ptr+len pairs as well as
NUL-terminated strings. This required removing default arguments
in the ConstBuffer constructor, since they cause nasty implicit
conversions.
TS-2957: add new sslheaders plugin

The sslheaders plugin injects information about the SSL session
into the client request, the server request or both. It can be used
to transmit client SSL information (eg, SSL certificate, certificate
subject, etc) to origin servers.
TS-2971 Change capitilization of authproxy debug symbols to make it consistent
TS-2973: add x-milestones header to the xdebug plugin

The X-Milestones header contains detailed information about how
long the transaction took to traverse portions of the HTTP state
machine. The timing information is obtained from the TSHttpTxnMilestoneGet
API. Each milestone value is a fractional number of seconds since
the beginning of the transaction.