fix error logging when index is locked on startup, add deprecation warning if unlockOnStartup is configured

LUCENE-6508 removed support for unlockOnStartup, but the way the changes are made are inconsistent with how other config options have been deprecated in the past, and cause a confusing error message if an index is locked on startup...

• in 5x, the SolrConfig constructor should log a warning if the unlockOnStartup option is specified (regardless of whether it's true or false) so users know they need to cleanup their configs and change their expectations (even if they never get - or have not yet gotten - a lock problem)

• in SolrCore, the LockObtainFailedException that is thrown if the index is locked should not say "Solr now longer supports forceful unlocking via 'unlockOnStartup'"
  • besides the no/now typo, this wording missleads users into thinking that the LockObtainFailedException error is in some way related to that config option – creating an implication that using that option lead them to this error. we shouldn't mention that here.