TS-2316: header_rewrite: added variable expansion support to add-header

Submitted by: Thomas Jackson <thjackso@linkedin.com>
Sponsored by: LinkedIn
TS-2316: header_rewrite: fix for crash in client header condition

Submitted by: Manjesh Nilange <mnilange@linkedin.com>
Sponsored by: LinkedIn
TS-2316: header_rewrite: added optional counter to each rule

Now you can count each rule's tripping by providing one or more "counter"
statements e.g::

    cond %{SEND_RESPONSE_HDR_HOOK} [AND]
    add-header X-DC "DC1"
    counter plugins.header_rewrite.POP
    counter plugins.header_rewrite.ALL

Submitted by: Alexey Ivanov <aivanov@linkedin.com>
Sponsored by: LinkedIn