New requirement when updating Team Services extensions on the Marketplace

Introduces a new requirement for updating Visual Studio Team Services extensions on the Marketplace: the version number must be incremented with each update. Explains the rationale behind this change (downstream problems with correlating versions and knowing the running version) and provides tooling options: the `tfx extension create --rev-version` argument to increment the patch segment, and build/release tasks that query the Marketplace for the current version, increment a specified segment, and use that new version for packaging.