There were two problems actually: the iomanager wasn't properly
deleting sourcesl; and in some situations, the remote serialize wasn't
registered with it to begin with.
Addresses BIT-1306 and probably also BIT-1356.
A bunch of infrastructure work to move IOSource, IOSourceRegistry (now
iosource::Manager) and PktSrc/PktDumper code into iosource/, and over
to a plugin structure.
Other IOSources aren't touched yet, they are still in src/*.
It compiles and does something with a small trace, but that's all I've
tested so far. There are quite certainly a number of problems left, as
well as various TODOs and cleanup; and nothing's cast in stone yet.
Will continue to work on this.
