[uploader] Avoid spinning on inbound requests

resultstore.go:
Changed receive on inboundRequestQueue to check for channel closure
by testing for a nil value (instead of ok-probing).
This effectively makes the channel block and wait for close().
Same in the for-loop that is paired with a periodic flush -- this
avoids polling.  As before, closing this
channel is the way to stop Run() gracefully.

utils_for_test.go:
fakeResultStoreServer now records RPC requests for comparison
in tests.

resultstore_test.go:
add wanted requests to test case structure.
add missing Serve() call.
add test case that excersises CreateInvocation only,
which looks like an interrupted build.
add test case with complete successful build.
Compare unpacked sequence of UploadRequests because batch size
can be variable and non-deterministic, due to timing.
Validate the continuity between authorization and resume tokens
in UploadBatchRequests.

Bug: 369980343

Change-Id: I79d449af28ba14759ceed1b19421b6ae9dad75f5
Reviewed-on: https://fuchsia-review.googlesource.com/c/rsclient/+/1360685
Commit-Queue: David Fang <fangism@google.com>
Reviewed-by: Jay Zhuang <jayzhuang@google.com>