Use byte slice for nextHop instead of string

This is because we're now writing the recipient into the pool file and
want a fixed length field (80 bytes).
Convert to processing on incoming, not outbound

Previously Yamn read mail, did minimal processing and stored it in the
pool.  Now it reads mail, does all possible processing and the pool
is mostly just a repository for outbound messages.
Create the concept of inbound and outbound pools

Inbound pool files are destined for the local remailer.
Outbound pool files are destined for other remailers.
Add inbound pool processing
Move message decoding to the server file.
Shuffle functions into more appropriate files
Minor logging change

Don't log about number of inbound messages processed unless at least one
has been processed.
Use envelope sender addy on remailer-foo requests
Remove an unwanted curly brace
Server need pubrings in order to randhop
Refine dummy processing and add it to client-side
Flush outbound pool on send_flag

If running with --send, flush the outbound pool.  This occurs is the
local node is a client or remailer.