NetDB: SubDB lifecycle fixes (Gitlab #406)

This should resolve the remaining portion of #406 (closed).

Tested both functionally and with VisualVM to verify that the FNDF objects do not leak after client/subdb shutdown.

major:

• Disable RefreshRoutersJob for subdbs, not required
• Cancel ExpireRoutersJob and ExpireLeasesJob at netdb shutdown
• Do not run ExpireRoutersJob at shutdown for subdbs, they are transient
• Do not create _exploreKeys HashMap for subdbs, and add checks for subdbs attempting to access
• Convert NegativeLookupCache timer to SimpleTimer2 so we may cancel it
• Add method to cancel NegativeLookupCache timer; cancel it when netdb shuts down

minor:

• Revert unnecessary subdb check in FloodfillMonitorJob, it is not instantiated for subdbs
• Revert unnecessary changes to KNDF.getDbDir() that were for reverted persistent subdbs, and add javadocs
• Revert changes in initialization of _dbDir in KNDF.startup()
• Disable and deprecate KNDF.restart(), it is unused, untested, and broken
• Remove DataStore.restart() and extensions, they are unused
• Cleanup unnecessary setting things to null that already are
• Cleanup in FNDF super.isMainDb() to simply isMainDb(); there is no override in FNDF to avoid

Not addressed here:

• Basic subdb creation/destruction (#406 (closed)) - fixed earlier
• Persistent subdb storage (#407 (closed)) - fixed earlier but still open
• Multihome DB (#409 (closed))
• Is burst lookup throttler required? (#453 (closed))

Closes #406 (closed)