Failures when auto-upgrading managed dependencies concurrently

[AnatoliB]

Two or more PowerShell workers may decide to auto-upgrade managed dependencies on the same Function app around the same time. As a result, they will be invoking Save-Module from different processes concurrently with the same target file system path. The problem is that Save-Module is not designed to handle this situation well:

• Some workers get warnings like "Access to the path ... is denied", often followed by a ProviderFailToDownloadFile error that terminates saving modules. This one could be mitigated by retries with random back-offs.
• Some workers do not get any failure indication from Save-Module, but the download process is not actually finished: some files are missing. This one is much worse as there is no easy way to detect that a retry is even needed.

Consider avoiding these issues by serializing auto-upgrade across all workers in the function app (use a file with a timestamp, a blob lease, etc. - TBD).

Alternatively, Save-Module could be fixed to handle concurrent invocations, or the PowerShell worker could install modules without using Save-Module, but these solutions look quite complicated.

[AnatoliB]

Addressed by #267