Bind CodecDataObject string instances

Bind CodecDataObject string instances

Heap analysis of the classes we generated on top of CodecDataObject
indicates we have an unnecessarily-large class constant pool. This
boils down our usage of string constants we pass back to
CodecDataObject -- which are coming from QName, but are not guaranteed
to be interned.

Aside from increasing heap consumption, this has a downside of not
requiring deep string comparisons during lookup -- which we want to
avoid if at all possible.

This patch updates generation strategy to create explicit constant
fields, which are bound to direct references to Strings used in
QNames -- resulting in the same objects being used.

JIRA: MDSAL-442
Change-Id: I10ef4ae49525d3f45076260e9438863b5830cf9a
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>