Optional support for file timestamps

Background

Optional support for file timestamps in NodeProperty. This is required for incremental workspace builds in BuildStream. For non-incremental builds (and sources not providing timestamps such as git kinds) the magic timestamps will continue to be used.

This depends on support in buildbox-casd: BuildGrid/buildbox/buildbox-casd#40 (closed)

This is considered DONE when:

• update protos (mirror changes in BuildGrid/buildbox/buildbox-common#32 and BuildGrid/buildbox/buildbox-casd#40 (closed))
• BuildGrid/buildbox/buildbox-common#32
• BuildGrid/buildbox/buildbox-casd#40 (closed)
• update docker image for latest casd
• Optionally support timestamps in importing trees to cascache:
  • support properties in CASCache.import_directory -> CaptureTree (casd implementation)
  • add node properties to _partial_import_cas_into_cas
  • if mtimes are requested, only copy files
    • properties support in IndexEntrys
  • optional on if source supports mtimes: BST_SOURCE_HAS_MTIME = True other sources should use magic times still
• Optionally support timestamps in staging trees from cascache
  • support properties in _import_files_from_cas and set mtime if supplied
  • support properties in export_files
    • parse MTime property in CASCache.checkout
  • sandboxes:
    • stage_sources ->
    • _stage_sources_in_sandbox ->
    • _stage_sources_at ->
    • sourcecache.export(source)
    • CasBasedDirectory.import_files
  • sourcecache.commit()
    • CasBasedDirectory.import_files
    • SourceCache.export
    • CasBasedDirectory.export_files
    • Source._stage
• test can stage with arbitrary mtime