Improve LoadTest extensibility

This patch rolls up related changes:

• Allow classes extending TestMiniClusterLoadSequential to override how reader and writer threads are set up, including extending the reader and writers themselves.

• Make it so classes extending MultiThreadedWriter can cleanly override how Puts are constructed.

• Provide an option for passing in a custom table descriptor to HBaseTestingUtility#createPreSplitLoadTestTable.

• HBaseTestingUtility#waitUntilAllRegionsAssigned does not check if the region it is counting belongs to the table created by the test and will not return if it accidentally counts "too many" regions, for example the regions of the ACL table when security is enabled. (Some class based on TestMiniClusterLoadSequential may want to enable security.)