fixes threading issue with context loading

[raleigh-g-thompson]

fixes #130

When calling getContext, from the vscode extension, the LSP4J creates multiple threads that call getContext(). If the resource isn't in the cache the IgContext goes searching. This is a big issue with the hover provider, it can causes multiple threads to go out and search for the same files, when the cache is cold. If IgContext can't find the file locally, or in .fhir/packages then it attempts to download from the npm registry. Once the cache is loaded, not an issue but when starting up it is a problem.

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Coverage Report

Overall Project	80.27%	🍏
Files changed	100%	🍏

Module	Coverage
CQL Language Server - Server Component	79.7%	🍏

Files

Module	File	Coverage
CQL Language Server - Server Component	IgContextManager.kt	53.31%	🍏

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.98%. Comparing base (5475d4f) to head (c7dd30a).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #132      +/-   ##
============================================
- Coverage     75.25%   73.98%   -1.28%     
+ Complexity      365      355      -10     
============================================
  Files            44       44              
  Lines          1160     1153       -7     
  Branches        171      167       -4     
============================================
- Hits            873      853      -20     
- Misses          189      210      +21     
+ Partials         98       90       -8     

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