Similar to lingering_time, it limits total connection lifetime before
keepalive is switched off.  The default is 1 hour, which is close to
the total maximum connection lifetime possible with default
keepalive_requests and keepalive_timeout.

If we need to be notified about further events, ngx_handle_read_event()
needs to be called after a read event is processed.  Without this,
an event can be removed from the kernel and won't be reported again,
notably when using oneshot event methods, such as eventport on Solaris.

While here, error handling is also added, similar to one present in
ngx_resolver_tcp_read().  This is not expected to make a difference
and mostly added for consistency.

If ngx_drain_connections() fails to immediately reuse any connections
and there are no free connections, it now additionally tries to reuse
a connection again.  This helps to provide at least one free connection
in case of HTTP/2 with lingering close, where merely trying to reuse
a connection once does not free it, but makes it reusable again,
waiting for lingering close.

Keeping post_accept_timeout in ngx_listening_t is no longer needed since
we've switched to 1 second timeout for deferred accept in 5541:fdb67cfc957d.

Further, using it in HTTP code can result in client_header_timeout being
used from an incorrect server block, notably if address-specific virtual
servers are used along with a wildcard listening socket, or if we've switched
to a different server block based on SNI in SSL handshake.