Adding a macro to define the vlog module in use adds a level of
indirection, which makes it easier to change how the vlog module must be
defined. A followup commit needs to do that, so getting these widespread
changes out of the way first should make that commit easier to review.
SSL, which will be added in an upcoming commit, requires some background
processing, which is best done in a "run" function in our architecture.
This commit adds stream_run() and stream_run_wait() and calls to them from
the places where they will be required.
This code is heavily based on the vconn code. Eventually we should make
the stream-based vconns (currently that's all of them) a wrapper around
streams, but I haven't done that yet.
SSL is not implemented yet.
