Replace existing gob-based merge impl with proto-based impl.

Merging now takes fully in C/C++ land with no callback to Go. Mildly
less flexible, but the the proto encoding is much more compact than the
gob encoding.
Address code review feedback.
Fix mem corruption in MergeOperator on the error path.
Add TODO about testing various merge error scenarios.
Merge pull request #120 from cockroachdb/pmattis/merge

Replace existing gob-based merge impl with proto-based impl.