Release 3.0.0-uber (2020-04-06)

Baseline: 3c7cc74

Cherry picks:

   + 4ee52db:
     Use prefix encoding for paths.
   + 9ada0fe:
     Avoid file operations in the sandbox creation critical path.
   + 80a2d7c:
     Implementation (but not plumbing) of the gRPC remote downloader
   + 586eabf:
     Implement RemoteDownloader w/ `--experimental_remote_downloader`
   + 2201827:
     Release 3.0.0 (2020-04-06)
   + eca40ab:
     BulkTransferException for remote up/download
   + aa700e7:
     Personal release scripts
   + 2e2edbe:
     Release 3.0.0-uber (2020-04-06)
   + 38895b8:
     Release 3.0.0-uber (2020-04-06)
   + 6b74783:
     Release 3.0.0-uber (2020-04-06)

This release contains contributions from many people at Google, as well as George Gensure, John Millikin.

CHANGELOG.md

@@ -23,6 +23,8 @@ Cherry picks:
      Release 3.0.0-uber (2020-04-06)
    + 38895b84f94adefca11a563df7df5ebcc7861db5:
      Release 3.0.0-uber (2020-04-06)
+   + 6b74783817f759b1990278359511c6716133fb6f:
+     Release 3.0.0-uber (2020-04-06)
 ```
 
 This release contains contributions from many people at Google, as well as George Gensure, John Millikin.