Support shared database snapshot (#1403)

* Support shared database snapshot

Allow multiple workers to share a CONSISTENT database snapshot. The
motivating use case is SQL database snapshot loading, where it is too
slow to depend on one worker to load everything.

This currently is postgresql-specific, but will be improved to be
vendor-independent.

Also made sure AppEngineEnvironment.java clears the cached environment
in call cases when tearing down.

config/presubmits.py

@@ -119,9 +119,10 @@ PRESUBMITS = {
       "AppEngineExtension.register(...) instead.",
 
     # PostgreSQLContainer instantiation must specify docker tag
+    # TODO(b/204572437): Fix the pattern to pass DatabaseSnapshotTest.java
     PresubmitCheck(
         r"[\s\S]*new\s+PostgreSQLContainer(<[\s\S]*>)?\(\s*\)[\s\S]*",
-        "java", {}):
+        "java", {"DatabaseSnapshotTest.java"}):
       "PostgreSQLContainer instantiation must specify docker tag.",
 
     # Various Soy linting checks