Drop AST for libraries after final analysis

[MikhailArkhipov]

Fixes #1088

Includes #1133

• Remove AST nodes from types
• Cache AST nodes at module level so they cache can be cleared when analysis is done.
• Occasionally analysis is requested again even when previous was claimed to be final, so add code to reload and reparse the file. This may be related to Stubs aren't analyzed before their modules #907.
• Keep AST for stubs generated by the scraper since some functions there require evaluation with arguments
• Remove reference to the AST root from nodes, reducing node size.
• Add qualified name to types in preparation for their identification in persistent models

[jakebailey]

One thing to check, CompletionTest.InImport is consistently failing for me.

 InImport
   Source: CompletionTests.cs line: 745
   Duration: 11 sec

  Message: 
    Expected completion list items to have label 'TestCase', but it has none.
  Stack Trace: 
    at LateBoundTestFramework.Throw(String message) in LateBoundTestFramework.cs line: 16
    at AssertionScope.FailWith(String message, Object[] args) in AssertionScope.cs line: 225
    at CompletionResultAssertions.HaveAttribute(IEnumerable`1 attributes, Func`2 attributeSelector, String itemNameSingle, String itemNamePlural, String because, Object[] reasonArgs) in CompletionResultAssertions.cs line: 113
    at CompletionResultAssertions.HaveLabels(IEnumerable`1 labels, String because, Object[] reasonArgs) in CompletionResultAssertions.cs line: 95
    at CompletionResultAssertions.HaveLabels(String[] labels) in CompletionResultAssertions.cs line: 91
    at CompletionTests.InImport() in CompletionTests.cs line: 789

   Open additional output for this result

[MikhailArkhipov]

Passes on 2017 and 2019, although there is some flakiness in tests that involve imports, I had 3 tests (different) failed but passed on rerun (re compile, datetime delta and some other). This is actually b/c 'analysis ready' event on the module is set when one of the sessions completes. Hence if module is analyzed before stub, as we have in the bug, analysis may end up incorrect as await returns with the analysis without the stub and not with the final one, which happens later. This happens in master as well.