Finalize support for invoking synchronization asynchronously

See the review items on the original ticket:
https://jira.magnolia-cms.com/browse/MGNLSYNC-40?focusedCommentId=208822&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-208822

Successfully merged basic functionality and released a 1.9-beta1; Here are additional review items for 2nd-stage review:

• REST API review - not a problem to change provided we produce a change log / release notes
• Rename 'repository' params back to 'workspace'
• Date resolution for the endpoint (help define expected ISO date string format?)
• Unbox booleans where applicable
• VH for module config and command bootstraps? non-applicable for new module
• Move manager to synchronization-core
• Builder for SynchronizationRequest + pass as param to #synchronize(SyncRequest), #canSynchronize...
• Remove XML output from endpoint (non-goal)
• Consider ITs for endpoint?
• Test method signatures: throw general Exception & no test prefix (BDD-style naming)
• Failing existing synchronization command tests – failing on JDK12, passing on JDK8
• Validate module config vs. manager, drop module lifecycle?
• Reconsider deduplicating `SynchronizationCommand` (preferably evolving or partially deprecating the old one)
• Global policy? //remove setter on manager, move the configuration under module config