Non-root prefix paths are mapped to fs incorrectly in Skylight

[MicrosecUjvari]

When a log has a non-root prefix path (ex. /subdir) this prefix path not stripped when URL mapped to localdirectory path in the filesystem.

For example if localdirectory is /opt/sunlight/logs/mylog/data and monitoringprefix is "https://myserver.example.com/monitor/mylog/" then /monitor/mylog/checkpoint request is mapped to /opt/sunlight/logs/mylog/data/monitor/mylog/checkpoint filesystem path instead of the expected /opt/sunlight/logs/mylog/data/checkpoint.

I think monitor URL prefix path should be stripped from request path when it is mapped to filesystem path.

[MicrosecUjvari]

Suggested fix: PR #58