Call setFeatureCompatibilityVersion once instead of for each database

Setting the featureCompatibilityVersion to 3.4 requires updating a document and creating an index. Setting the featureCompatibilityVersion to 3.2 requires updating a document and dropping that index. The validate_collections.js hook sets the featureCompatibilityVersion to 3.4 for each collection when running the fuzzer. This causes additional I/O load on the machine and can lead to the test being considered as having timed out. We should instead change to set the featureCompatibilityVersion to 3.4 once while validating all collections on the server before setting it back to 3.2.