Updates to InboxMessage behavior

[armiol]

This changeset modifies the way we store InboxMessages.

• The timestamp of InboxMessage now has microsecond precision. On storage level, it allows using the native data types, which typically support microseconds at most.
• The identifier of InboxMessage now includes the ShardIndex which previously was a part of InboxMessage body. Such an approach allows storing InboxMessages as a hierarchy of objects. In turn, in Google Datastore that opens a possibility to query these messages in a transaction.

Please note that existing binaries of InboxMessage become incompatible with this code version.
So make sure to deliver or purge the existing messages of this type prior to migration.

As a minor change, the CatchUpTest has been made open for configuration in the dependant libraries, such as spine-gcloud-java.

The library version is updated to 1.4.9.

[codecov]

Codecov Report

Merging #1239 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #1239      +/-   ##
============================================
- Coverage     91.22%   91.21%   -0.02%     
- Complexity     4726     4728       +2     
============================================
  Files           615      616       +1     
  Lines         14823    14828       +5     
  Branches        834      834              
============================================
+ Hits          13523    13526       +3     
- Misses         1044     1046       +2     
  Partials        256      256

[dmdashenkov]

@armiol, LGTM with a couple of formatting comments.