NEXUS-5472: Separated update of repo types

Into different methods, as only proxy discovery will probably
need to happen async. Others are "fast" to make them sync.

At least group update is fast for sure, for hosted with huge
content we might see, do we want to make it async or sync
NEXUS-5472: Cleanup.

Removed unused imports and updated javadoc
NEXUS-5472: Fixed DTO and JS wrt Int/Long conflict.

And made field names explicitly show interval is defined in hours.
NEXUS-5472: Added UT for local discoverer.
Fix potential NPE
UT for remote content discovery