java/syncbase: Improve AccessList inspectability and Watch Cancel

We improve usability of the AccessList and Watch.
The former can be inspected more easily now.
And the latter can be canceled without throwing.

Note: We will want to actually implement watch cancel correctly.
Change-Id: Ib8f4f7a2516342c537e9cda6ad96f5a33e147121
4 files changed