p2p network txn handling, new constants, socket options

[pmesnier]

Relates to #291, cleans up txn handling, define constants for new magic numbers, add setting the NODELAY socket option.

[pmesnier]

I've named several magic numbers in net_plugin.cpp. This adds 9 potentially configurable values to the four existing. I think it would be a good idea to add either section names or prefixes to config options to avoid potential namespace collisions. For instance, change listen-endpoint, public-endpoint, and remote-endpoint to be p2p-listen-endpoint, etc. then the new values when added could be p2p-.

In fact, we don't need to do that to all config.ini options, http-server-endpoint is clear, as is plugin and producer-name.

[pmesnier]

closing, solution was merged in pr #403.