Implement speculative queue

Such a queue does not wait for actual queue and client initialization,
and in a mean time stows all records in memory. Once the scheduled
actual queue initialization succeeds, all stowed records are enqueued.
Note that stowed records will be lost if the machine is rebooted. They
are not encrypted in memory, until enqueued to the actual queue.

The code sample for using it is placed in the comment in report_queue.h

Note also, SpeculativeQueue implementation has weak pointers factory
so its 'Create' method which is used for instantiating it returns
a smart pointer augmented with on-thread deleter!

Bug: 185173853
Change-Id: I652fe823c8ffa73110c4c13262bc3ee7dc1ef5ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3069666


Commit-Queue: Leonid Baraz <lbaraz@chromium.org>
Reviewed-by: Zach Trudo <zatrudo@google.com>
Cr-Commit-Position: refs/heads/master@{#908708}