rtmpsrc: fix flushing seek

rtmpsrc: fix flushing seek

Previously this was broken, because a flushing seek causes unlock()
to be called and in the implementation of unlock() we close the
socket, so the seek errors out.

This patch fixes it by re-connecting before the seek.
Unfortunately, a seek does not work properly right after
re-connecting, so a small hack is also in place: we read 1 buffer
before seeking to allow librtmp to do its processing in RTMP_Read()

https://bugzilla.gnome.org/show_bug.cgi?id=785941