Use pvxslibs

[mdavidsaver]

My attempt to switch from pva2pva to pvxs. Also adds a way to execute any IOC shell command. Needs pvxslibs 1.2.1a1, specifically https://github.com/mdavidsaver/pvxs/commit/a1eb49386f3b3e7c8a3884413275e32e7ac58402 .

$ python3 -m softioc ../docs/examples/example_asyncio_ioc.py
INFO: PVXS QSRV2 is loaded and ENABLED.
Starting iocInit
############################################################################
## EPICS 7.0.7.0
## Rev. 7.0.7.99.0.0a1
## Rev. Date 7.0.7.99.0.0a1
############################################################################
iocRun: All initialization complete
Python 3.9.2 (default, Feb 28 2021, 17:03:44) 
[GCC 10.2.1 20210110] on linux
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> iocsh.epicsThreadShowAll()
            NAME       EPICS ID   LWP ID   OSIPRI  OSSPRI  STATE
          _main_      0x26f06b0        6      0       0       OK
          PVXTCP      0x2b9c570        9     18       0       OK
          PVXUDP      0x2badac0       10     16       0       OK
...

[codecov[bot]]

Codecov Report

Merging #133 (bc990c7) into master (289897e) will decrease coverage by 1.03%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #133      +/-   ##
==========================================
- Coverage   87.44%   86.42%   -1.03%     
==========================================
  Files          14       14              
  Lines         996     1031      +35     
==========================================
+ Hits          871      891      +20     
- Misses        125      140      +15     

Impacted Files	Coverage Δ
softioc/softioc.py	71.64% <44.00%> (-6.34%)	:arrow_down:
softioc/__init__.py	95.83% <95.00%> (+2.97%)	:arrow_up:

... and 1 file with indirect coverage changes

[coretl]

Closed in favour of #132