Support pre-commit callback

[mikee47]

This PR provides support for registering callbacks on objects, invoked just before changes are committed. Typically this would be at the end of a streaming import.

Applications call onCommit to register a callback, which remains active indefinitely. The mechanism is similar to how asynchronous updates are handled, except those are 'one-shot' callbacks.

The callback receives an updater instance, and if clearDirty is called then the data is never flushed to the filesystem. This can be used to support 'ephemeral' or temporary in-RAM only use.

Another use for the callback is to perform extended data validation before data is written to the filesystem. The update can be rejected by calling clearDirty.

Change callbacks are discussed in issue #48. This PR is a partial solution to that but at present there is no indication as to which fields have changed.

TODO:

• Use a single queue for asynchronous updates and the commit callbacks, and tag the items so other types of callback may be registered in the future
• Update documentation
• Look at memory usage in commit callback: ideally, allocated storage for streaming should be released beforehand