Added support for partial consumer mode

[EmilianoSanchez]

Javascript commons library

What did you accomplish?

Added support for partial consumer mode:

• Updated PluggableStorage to work with in memory versions of events and impressions cache, and support optimized mode for imp. dedup.
• Updated SyncManager to be instantiated with submitters but not polling manager.
• Updated mode validator, to address the new mode type 'consumer_partial'.
• Updated storage validator, to throw error on invalid combinations of mode and storage type (e.g., consumer mode requires custom/pluggable storage).

How do we test the changes introduced in this PR?

• Updated tests

Extra Notes

TODO: review mode & storage defaults and validation