http: supported chunked Transfer-Encoding

This change implements support for HTTP Transfer-Encoding
chunked in both JSON and Form (post vars) body content. A
new function ast_http_get_contents() handles both regular
and chunked mode body, returning after the entire body is
received.

(closes issue ASTERISK-23068)
Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/3125/
........

Merged revisions 405861 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@405862 65c4cc65-6c06-0410-ace0-fbb531ad65f3