remove ngx_http_push_pool to avoid memory leaks

Nginx pool allocator is meant to be used for storing stuff while handling the HTTP request
and is usually destroyed when the request is done. Since the allocator assumes the pool is
short lived and will be destroyed anyway it really doesn't keep track of small allocations
and just takes new memory from the internally allocated blocks.
So calling ngx_pfree() on a small allocation (smaller than 4KiB by default) doesn't really do anything.

Since ngx_http_push_pool was a long lived pool that was created on module init it lead to memory leaks
even though ngx_pfree() was called correctly.
This patch basically just rips out pool allocator usage and replaces it with system allocator usage.
(cherry picked from commit 0196b15d63f210694bdbebe3a03ea4a86396e59a)
remove unused variables. only removing the assignment of rc because i'm not sure if ngx_http_push_broadcast_status_locked has side effects.

see ticket: https://github.com/slact/nginx_http_push_module/issues/#issue/64/
Prevent leaking a nginx worker connection every time a client disconnects before the request has been fulfilled. We were running out of worker connections every hour or so!
fix for "push_subscriber_concurrency last"

push_subscriber_concurrency last didn't work quite as expected: older connections
did indeed get HTTP 409 Conflict when a new subscriber arrived but after that the new
connection was terminated without returning any status code.

This was caused by broken check for return value of the ngx_http_push_broadcast_status_locked()
which is just a wrapper for ngx_http_push_broadcast_locked() which in turn can only return
NGX_HTTP_PUSH_MESSAGE_RECEIVED or NGX_HTTP_PUSH_MESSAGE_QUEUED so the response was always
interpreted as error.

Conflicts:
	src/ngx_http_push_module.c
Merge branches 'conn_leak', 'hadaras_pick' and 'build_warn'