* Actually drop the connection when no response to ping arrives within the next interval.
* Avoid sending immediate ping when wall time is adjusted, it's usually spurious.
PUBLISHED_FROM=8049280b58edfb94dd0fcb6a1e89ffefe69bcea1
If user throttles receive by setting recv_mbuf_limit,
after the net interface reports connection as closed we must wait
for data to trickle through before disposing of it.
There can still b data in the buffers (e.g. SSL).
CL: mg: Ensure that user sees all the data before connection is closed
PUBLISHED_FROM=22be0fa368950a9fdb03cfb00febc7c0a1674b01
* Change return type of mg_mgr_poll to return number of events
* Add mg_mgr_min_timer
* Refactor main poll loop to remove LwIP-specific stuff
CL: Refactor mg polling
PUBLISHED_FROM=dc94618b32fa3c84a2f053bd04d134297780ec82
