Stricter configuration validation

[ArbaazKhan1]

Closes issue #6216

• Add new Property Type: CLOSED_PREFIX to represent non-extensible namespace prefixes
• The existing PropertyType.PREFIX remains open for user-extensible properties (e.g. table.custom, table.iterator ...)
• Reclassified tserver, manager, gc, monitor, rpc, sserver, compactor, compaction-coordinator, and instance to CLOSED_PREFIX
• updated unit test to test for new closed prefix type
• Updated ConfigCheckUtil, AccumuloConfiguration, ConfigurationImpl, ConfigurationDocGen, and SystemPropUtil to handle CLOSED_PREFIX wherever PropertyType.PREFIX was previously checked
• Fix DefaultConfiguration static initializer to exclude CLOSED_PREFIX properties, which also have null default values
• Unrecognized keys under closed prefix now log a warning instead of silently being accepted

[ArbaazKhan1]

I know PR #6487 is also working on updating Properties. It looks like the work done there v here is mostly orthogonal, but depending on which get merged in first there will likely be some merge conflict, since they both touch the same block in Property.java. Worth double-checking once #6487 settles: if it ends up marking a property "required" just because its default value is null, that would wrongly flag every prefix property as required too, since both PREFIX and CLOSED_PREFIX types always have a null default by convention.