exchange: add the pushmarker wireproto command to simple4server This will allow simple server side support. (yes, code duplication is bad, I won't do it too much mom)

exchange: introduce a wireprotocol command to push markers Pushkey is not adapted to transmit a stream of binary data. We used it as a hack so far. We add a new wire protocol command to speed the things up (and also push all the markers in a single transaction). The proper way to do it is to introduce bundle2 in core, but this will already help in the mean time.

exchange: actually use _pushkeyescape fallback The code was still calling `obsolete._pushkeyescape` therefore never calling the fallback we defined. Now that we call it, it had to be fixed to actually work too.