* cli: added a flag to create repository with v2 index features
* content: plumb through compression.ID parameter to content.Manager.WriteContent()
* content: expose content.Manager.SupportsContentCompression
This allows object manager to decide whether to create compressed object
or let the content manager do it.
* object: if compression is requested and the repo supports it, pass compression ID to the content manager
* cli: show compression status in 'repository status'
* cli: output compression information in 'content list' and 'content stats'
* content: compression and decompression support
* content: unit tests for compression
* object: compression tests
* testing: added integration tests against v2 index
* testing: run all e2e tests with and without content-level compression
* htmlui: added UI for specifying index format on creation
* cli: additional tests for 'content ls' and 'content stats'
* applied pr suggestions
* grpcapi: added GPRC API for the repository server
* repo: added transparent retries to GRPC repository client
Normally GRPC reconnects automatically, which can survive server
restarts (minus transient errors).
In our case we're establishing a stream which will be broken and
needs to be restarted after io.EOF is detected.
It safe to do transparent retries for read-only (repo.Repository),
but not safe for write sessions (repo.RepositoryWriter), because the
session may re-connect to different server that won't have the buffered
content write available in memory.
