Port all WorkMgrV1 calls to WorkMgrV2 calls when available

[arifogel]

• add version check on Session initialization to determine whether to use only v2 calls or use legacy v1 calls
• can force behavior without query by setting environment variable pybf_use_deprecated_workmgr_v1 to 0 or 1, or by passing use_deprecated_workmgr_v1=True/False to Session.__init__

[batfish-bot]

This change is 

[codecov[bot]]

Codecov Report

Merging #851 (a5b004a) into master (c8b54d8) will increase coverage by 0.32%. The diff coverage is 88.59%.

@@            Coverage Diff             @@
##           master     #851      +/-   ##
==========================================
+ Coverage   86.23%   86.56%   +0.32%     
==========================================
  Files          30       30              
  Lines        3851     3951     +100     
  Branches      644      664      +20     
==========================================
+ Hits         3321     3420      +99     
+ Misses        387      384       -3     
- Partials      143      147       +4     

Impacted Files	Coverage Δ
pybatfish/question/question.py	82.62% <ø> (ø)
pybatfish/client/session.py	80.88% <81.69%> (+1.25%)	:arrow_up:
pybatfish/client/workitem.py	84.21% <88.88%> (+1.85%)	:arrow_up:
pybatfish/client/workhelper.py	76.63% <90.00%> (+3.48%)	:arrow_up:
pybatfish/client/restv2helper.py	70.52% <97.22%> (+3.85%)	:arrow_up:
pybatfish/client/consts.py	100.00% <100.00%> (ø)
pybatfish/client/internal.py	83.33% <100.00%> (ø)
pybatfish/client/resthelper.py	75.00% <100.00%> (+0.58%)	:arrow_up:

[arifogel]

a discussion (no related file): Adding a new batfish endpoint for getting api version. Will switch over once merged.