SERVER-8046 Remove some impossible error condition checks

If a node N has a parent, and you are trying to add a sibling to N, then N is already a child,
and therefore N's parent is provably not a leaf. Remove the redundant error checks for this
condition, but leave them as a dassert, at least for now.
SERVER-10142 Fix windows warning for cases where dassert is compiled out
SERVER-8046 A few additional coverage driven unit test updates
SERVER-8046 Validate values during type encoding tests
SERVER-8046 Cleanup Element::writeArrayTo and add a test for the new implementation
SERVER-8046 Account for field name NULL terminator when computing damage offsets