Signed-off-by: Viktor Söderqvist <viktor.soderqvist@est.tech>

Adds event_handler and event_handler_state in the map returned by
gun:info/1.

Signed-off-by: Viktor Söderqvist <viktor.soderqvist@est.tech>

LH: I have expanded and documented gun_notify in its own page.

LH: I have fixed types, extended tests and tweaked a bit.

The number of unacknowledged pings that can be tolerated
before the connection is forcefully closed.

When a keepalive ping is sent to the peer, a counter is
incremented and if this counter exceeds the tolerance limit,
the connection is forcefully closed. The counter is
decremented whenever a ping ack is received from the peer.

By default, the mechanism for closing the connection based
on ping and ping ack is disabled.

Loïc Hoguin: I have edited a lot of the code and renamed
a few things as well as simplified the docs and increased
test timeouts to avoid race conditions.

When no TCP options are provided, Gun will enable send
timeouts at 15s. The value was chosen large enough to
be safe while still allowing Gun to detect send errors
eventually. Different applications may need to tweak
and lower this value.