Today we rely on blob stores behaving in a certain way so that they can be used
as a snapshot repository. There are an increasing number of third-party blob
stores that claim to be S3-compatible, but which may not offer a suitably
correct or performant implementation of the S3 API. We rely on somesubtle
semantics with concurrent readers and writers, but some blob stores may not
implement it correctly. Hitting a corner case in the implementation may be rare
in normal use, and may be hard to reproduce or to distinguish from an
Elasticsearch bug.
This commit introduces a new `POST /_snapshot/.../_analyse` API which exercises
the more problematic corners of the repository implementation looking for
correctness bugs and measures the details of the performance of the repository
under concurrent load.
