Restore OTLogStream::overflow, removed in commit 7deb336.

It broke the opentxs output to stdout/stderr (e.g. "opentxs list").
overflow is an inherited function from streambuf, which OTLogStream
inherits from.
Also:
Revert "OTLog: -Wunused-private-field"
This reverts commit d09d187767eb251d1f8ff91bb1730dfe0a5fe7e2.