mod_ssl: coalesce using a bucket brigade and the setaside/reinstate mechanism.

mod_ssl: coalesce using a bucket brigade and the setaside/reinstate mechanism.

ssl_io_filter_coalesce() now uses apr_brigade_write() to save its retained data
in a heap bucket, and ap_filter_{setaside,reinstate}_brigade() to declare them
to the output filters' write completion mechanism.

This prevents MPM event to miss them when it enters write completion state, and
will allow the tunneling loop of mod_proxy to flush them in a following commit
too.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1889938 13f79535-47bb-0310-9956-ffa450edef68