Leave upload mode is too long. Proposed patch

Hi,

we've faced with following unexpected behavior, after leaving upload
mode torrent download started after really long time.
Investigation shows that behavior is following:
- when torrent switched to upload only mode it periodically (default
is 100 sec min_reconnect) reconnect to peers if they are seeds or
upload onl  do nothing for 20 sec (as it is upload mode) then
connection dropped by remote side with ' no_request' error, so remote
and local resources used for nothing (socket, connection, etc.).
As connection closed peer last_connected time is shifted and next
connect to this peer will be after 100 sec timeout.
So when leaving upload mode torrent download will start only when 100
sec timeout exceeded after last connection to peer in upload mode, or
even worse ~= 20 + 100 sec for connection which were opened in upload
only mode when we leave it as for some reason this connections is not
reused for download when we leave upload mode.

There are the way to set min_reconnect timeout, but this is bad in
general as we will force faster reconnects even to bad peers.
There are also fast_reconnect flag on peer_connection but it is not
suitable too, as we have no connections in upload mode most time, we
have only peers.

Attached patch resolve fast start of download after leaving upload
mode (without touching existing behavior with timeouts) as following:
* it reset last_connected timestamp when leaving upload mode, to force
peer connection on nearest session ticks;
* it close periodical connection in upload mode if remote side also in
upload mode (or seed) with upload_to_upload error, as it is done when
two seeders connected to each other, so connections and resources are
not used for nothing (20 sec connection lock without real usage seems
too long for high performance seed for example).

We are using libtorrent 0.15.4, but patch is suitable for trunk too.

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