Cache Issue Tracker connections per base_url. Fixes #290

[atodorov]

in this way we create object to connect to the remote end only
once, which should improve speed of work when working often with
bugs directly from Kiwi TCMS. This also makes it possible to
still have multiple IT integrations in the same instance b/c they
will have unique base URLs.

[coveralls]

Pull Request Test Coverage Report for Build 3850

• 9 of 21 (42.86%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 73.467%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tcms/issuetracker/types.py	9	21	42.86%

Totals
Change from base Build 3786:	0.08%
Covered Lines:	3580
Relevant Lines:	4603

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💛 - Coveralls

[codecov-io]

Codecov Report

Merging #1093 into remove_bug_use_linkreference will increase coverage by 0.06%.
The diff coverage is 42.85%.

@@                       Coverage Diff                        @@
##           remove_bug_use_linkreference    #1093      +/-   ##
================================================================
+ Coverage                         73.06%   73.12%   +0.06%     
================================================================
  Files                               111      111              
  Lines                              4603     4603              
  Branches                            591      590       -1     
================================================================
+ Hits                               3363     3366       +3     
+ Misses                             1026     1022       -4     
- Partials                            214      215       +1

Impacted Files	Coverage Δ
tcms/issuetracker/types.py	32.16% <42.85%> (+1.75%)	⬆️
tcms/core/history.py	90.69% <0%> (ø)	⬆️

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