Add a sendRcon method to BaseInstancePlugin and the slave's Instance
class. This servers two purposes, one is to reduce the repetition when
send RCON commands particularly from plugins, and the other is to
instrument the RCON commands sent with metrics about duration and size.
Use lib* prefix for the modules impored from the lib package to help
distinguish them more from ordinary variables. Fix up the naming
convention for files to be consistently lowercase_underscore for
JavaScript files.
Split up codebase into the packages @clsterio/lib, @clusterio/master,
@clusterio/slave, and @clusterio/ctl using lerna to manage them. This
will allow deployment to use only the parts needed reducing install
size, and better separate the code into shared library, master, slave
and control.