Add the uSync.Once and uSync.Stop functionalty in for v8

[KevinJump]

in previous versions of uSync if there is a usync.once file in the import folder then upon import completion, that file is replaced with a usync.stop file.

at startup, if there is a usync.stop the file then the imports do not happen.

this lets you have a 'single time-only' import mechanism, which can be good for a developer flow when initializing a repo, or for deployments where you only want it to happen the very first time after deployment.

[KevinJump]

commit #9b0b818bb935fda70e1d3b742cd30a7074983e68 will be in next release (v8.2.4)