txaio.time_ns requires networking framework selection

[oberstet]

The WAMP IDL code generator (xbrnetwork CLI included with Autobahn) currently generates code that imports txaio and select Twisted. When writing asyncio programs, this is inconvenient. The problem is time_ns:

(cpy392_1) oberstet@intel-nuci7:~$ python
Python 3.9.2 (default, Mar  8 2021, 02:05:12) 
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from txaio import time_ns
>>> time_ns()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/oberstet/scm/crossbario/txaio/txaio/_unframework.py", line 41, in _throw_usage_error
    raise RuntimeError(
RuntimeError: To use txaio, you must first select a framework with .use_twisted() or .use_asyncio()
>>> 

We either need a way to use txaio.time_ns without selecting a networking framework, or we need to move the helper eg to zLMDB.

[meejah]

Kind of the whole premise of txaio is "select things via imports" .. now, those are usually like the autobahn.wamp.{twisted,asyncio} method (the .use_*() calls are in there) but ... might be expedient to just move / duplicate it..?

...but also, it seems it's only there to support Python versions less than 3.7 -- above that it's time.time_ns()