Polish user interfaces of texts (script and session)

• The flag -a, --async-run does not make much sense for texts script unless the sleep_time of the launcher thread of the engine is much smaller than the default 30 seconds. And there is no access to sleep_time through the texts interface. And even in the case it were there and a very small meaningful sleep_time is chosen (like 1 second) then the execution becomes nearly "synchronous" though not guarantee to finish before the interpreter is finished in the main thread and a stop is sent to the launcher thread.

• We can provide a flag to change sleep_time in the "texts session". Of course, we can also provide a magic to change the time interactively in the session (similar to %start and %stop).

• The (lack of) --no_unique_launchdir is currently not honored by the default launcher that without -a runs in the main thread after the workflow is added to the database. This has to be fixed.

• The docs should include an explanation of the behavior and the default policies (unique launch directories for interactive nodes and no-submit policy for batch nodes in synchronous mode) behind in simple wording, with examples.