Stock evhttp has no means to detect when an incoming connection gets
closed by the client; it will notice the connection has gone down only
when sending back a reply.
For DAAP update requests working as a push mechanism with an HTTP request
stalled by the server until there actually is an update available, we need
to be notified when a connection goes down so we can perform proper cleanup
and not retain memory.
Do so by extending the close detection mechanism used for outgoing connections
and the connection failure callback we already have in place for streaming.
Get rid of strlcpy() and its implementation entirely, it doesn't buy anything
over snprintf(). Use evutil_snprintf() so as to match the rest of the code.
evhttp_write_buffer() used by evhttp_send_reply_chunk() can take callback
executed when (part of) the buffer has been written. Using this callback to
schedule the next chunk avoids buffering large amounts of data in memory.
The callback will be executed if a failure (timeout, read error) is detected
on an incoming connection.
This can be used to detect a client closing the connection while engaged in a
chunked reply and actively pushing chunks to the client. Without this
notification, it is impossible to know if/when a connection gets closed and
react appropriately (ie. stop pushing chunks) resulting in a segfault.
