Sst rollup branch

Add a better "simple evpath" dataplane than the old "dummy" dataplane. The latter remains as an example, but the former uses sockets (rather than RUDP) for data connections, making it more useful for the WAN situation. Create a DataTransport parameter so that we can specify the dataplane to load from the ADIOS2 level. Add a preliminary reader-close protocol so that the writer doesn't hang forever if the reader doesn't read all the data. Fix a race condition in writer-close that caused the reader to mess that event sometimes.