Use memoized suppliers to lazy load resources

[ctubbsii]

• Remove the need to synchronize on reads for lazily initialized
  singleton resources, particularly in the configuration and context
  utilities, using Suppliers.memoize
• Apply to DefaultConfiguration.getInstance() to avoid unnecessary
  object creation whenever that is called
• Make all ServerContext fields final, and use memoize to lazily load
  anything that was previously checking if it was set in a synchronized
  getter method
• Use computeIfAbsent in ServerConfigurationFactory, and make its caches
  of configuration objects non-static, so they only live as long as the
  ServerContext that created it lives; this, along with using
  ConcurrentHashMaps, dramatically simplifies this code
• Reduce ServerContext reliance on ServerConfigurationFactory when it
  already has the information (notably, it has the instance of
  SiteConfiguration it used to construct the ServerConfigurationFactory)
• Add a builder option for SiteConfiguration.empty() for use with tests
• Update related tests, and make ServerContextTest.testCanRun more
  robust

[milleruntime]

Were these changes pulled from the ZK single node prop store refactor?

[EdColeman]

Yes - the goal is to submit changes that are adjacent to the single node changes and have them submitted separately to reduce the number of absolute changes in the single node PR. It also contains changes that @ctubbsii identified during his review that were not originally in the single node PR, but are general code improvements and were in areas touched by the single node PR

If these changes are merged, the the single prop node PR can be overlaid on top of them with work that is being done as part of his review.