Fix some thread safety issues and improve concurrency tests

[JWCook]

Related to #515.

Tests

• Add stress test shortcut for local development (nox -e stress)
• Use fewer unique responses and more total requests to increase concurrent cache hits
• Add multiprocess tests (in addition to multithreaded tests)

Fixes

• Handle CorruptGridFile error in GridFS backend
• Fix race condition with SQLiteDict.clear(), when dropping and recreating SQLite tables in multiple threads
• Fix some additional file-based race conditions in filesystem and GridFS backends
  • Note: These still do not implement a full file locking system, and aren't ideal for highly parallel applications
• Fix potential race condition in remove_expired_responses() for SQLite, Filesystem, and GridFS backends

[codecov-commenter]

Codecov Report

Merging #537 (15e1154) into master (6e595f3) will decrease coverage by 0.12%.
The diff coverage is 95.23%.

@@            Coverage Diff             @@
##           master     #537      +/-   ##
==========================================
- Coverage   98.54%   98.41%   -0.13%     
==========================================
  Files          18       18              
  Lines        1303     1325      +22     
  Branches      207      216       +9     
==========================================
+ Hits         1284     1304      +20     
- Misses         10       12       +2     
  Partials        9        9              

Impacted Files	Coverage Δ
requests_cache/backends/gridfs.py	96.29% <91.66%> (-3.71%)	⬇️
requests_cache/backends/filesystem.py	96.05% <100.00%> (+0.40%)	⬆️
requests_cache/backends/sqlite.py	98.72% <100.00%> (+0.03%)	⬆️

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