[Database] Show warning if persistence seems unavailable

Show a warning dialog to the user if the persistence service is taking abnormally long (10 seconds or more) to become available; this may indicate that the database is unavailable, credentials are wrong, etc. This addresses nasa/mct#225

     Wrote or re-wrote unit tests (Y/N): N
     Ran entire suite of unit tests successfully (Y/N): Y
     Verified bug fix with the described replication steps or verified the implementation meets the requirement cited (Y/N): Y
     Code coverage requirements satisfied (new code=80%, changed code=at least as high as existing) (Y/N): N
     Code review date and attendees: Self-review
     Describe potential impact on other areas of the code: Database, startup
     Describe any test prerequisites for this fix (e.g., restart the database): Configure persistence.properties to point to an unavailable or nonexistent database
     For bugs marked "Regression - Yes", describe the cause of the regression: N/A
     Did you modify any pom.xml file? If yes, did you thoroughly document your changes and reasons? N

Unit tests not included. New code is primarily in a one-minute timer; exercising this would inflate time taken to run test suite without making a meaningful improvement in code coverage.