OPENDJ-2776 Remove insignificant/illegal whitespace during parsing of DNs

Strictly speaking RFC 4514 does not allow whitespace before or after RDN
or AVA separators. Whilst it is often good practice to be tolerant of
malformed input, we should not produce it.

The DN class was tolerating whitespace characters around the RDN and AVA
separators, but was not removing them, causing the toString() method to
product an invalid string representation.

This change provides a simple fix: do not cache the user provided
string, but instead regenerate it lazily when toString() is called.
Whilst the fix is simple it does incur a slight performance penalty in
the common case where the input DN string is valid.