Add NetCoreApi compatability to uSync.Complete v8

[KevinJump]

For uSync.Complete on .NetCore there are some very subtle differences in how messages are autenticated. left un changed this would be the only issue stopping v8 -> v9 communication. so we are back porting this change to v8.

If we change v8 to sign messages the same way as v9 we would break all existing v8 installations, so the change will be hidden behind an option.

adding a useCoreApi option to the usync.publisher.config fille will turn this feature on, and v8 sites will be able to communicate with v9 sites.

  <useCoreApi>true</useCoreApi>

once this change has been made a v8 + useCoreApi site will no longer be able to push to a v8 - useCoreApi site. (so all sites will need the setting on if you wish to communicate between v8 and v9 sites.

for now this setting will be false - we may change that in a later (breaking) release.