Improve handling of partial keys

[ddeclerck]

This PR improves handling of partial keys, to ensure BDB always compares keys of identical length (which would trigger an error when using a non-native collating sequence, since the custom comparison function enforce keys of same length).

[GitMensch]

That looks clean - let's get that upstream!

[ddeclerck]

Just for you to know I ran some perf test on this, and my findings are as follows:

• I measure similar performance with and without this patch when using BDB's default comparison function
• when using the custom comparison function, this patch (which uses indexed_key_compare to compare keys of identical length) yields better performance than a non-patched version that uses a comparison function that considers keys of different lengths (cob_cmp_strings)

So this would confirm this patch is the right approach.

As you already approved this, I'll merge upstream in a moment.

[ddeclerck]

Merged in SVN @ revision 5412.