Replace package `lockfile` with internal mutex implementation.

[smithsz]

Checklist

• [x] Tick to sign-off your agreement to the Developer Certificate of Origin (DCO) 1.1
• [x] Added tests for code changes or test/build only changes
• [x] Updated the change log file (CHANGES.md|CHANGELOG.md) or test/build only changes
• [x] Completed the PR template below:

Description

Replace package lockfile with internal mutex implementation.

Approach

The new Mutex class provides a basic lock that can be used to prevent the simultaneous access to a resource.

Schema & API Changes

No change.

Security and Privacy

No change.

Testing

Includes additional unit testing.

Monitoring and Logging

Adds a new log class cloudant:mutex.

[smithsz]

This isn't quite right. It's a lot faster than lockfile but soon exceeds the stack size limit if there's a lot of concurrent lock acquire attempts. Back to the drawing board!