net.box: rename netbox_{prepare,encode}_request to {begin,end}

netbox_encode_request is a confusing name - the function doesn't encode
a request, it just writes the actual request size to the fix header.
Let's rename the two functions to netbox_begin_request and
netbox_end_request to emphasize that they are used to start and finish
request encoding.

Another reason to do that now is that in the following patches we will
be moving the request table implementation from Lua to C so having a
"request" in the function names would be confusing.

Part of #6241