Fixes WT-1791

There are a few changes:
1. Use beginthreadex to allocate CRT TLS structures at thread creation instead of deferring their allocation. This is also better for Managed C++.
2. Ensure all APIs use *stdcall and *cdecl as appropriate. By default under x64 MSVC this does not make a difference since *stdcall == *cdecl unlike on x86.
3. Add a build variant to MCI system that ensures we use the correct calling conventions everywhere by forcing the default calling convention to __vectorcall. This ensures any missing calling convention annotations are build failures.

Note: I cannot compile the tests, and python wrapper with the stronger warnings so I only verify calling convention compatibility against wiredtiger itself.

Tests:
Windows with and without /Gv