pemfile: Move file watcher plugin from advancedtls to gRPC.

[easwars]

This PR moves the existing PEM file watcher certificate provider plugin from the advancedtls module to gRPC.

The only changes of significance are:

• Check the contents of the file to see if it has changed before updating distributor.
• Update the tests.

[easwars]

I had done a git mv and initially it was showing as though the file had been branched from its earlier location. But now it shows up as being deleted at the old location and added at the new location :(

[easwars]

This is getting a little out of control to rebase with master after #3932. I will close this and reopen a new PR.