Initial roll-on-locked implementation for issue #93.

Detects file locking when opening rolling files, and if the file is locked, inserts a `_001`-style sequence number into the filename after the `{Date}` token.

To avoid the "next" opener writing to the original file after an increment is created, this change requires searching for existing increments each time a new file is opened. This is where any risk of destabilizing current rolling behavior will lie.

For #93.