Removed Principal object from ObjectApprovers class.
Removed Principal object from ObjectApprovers class.

After creation on an `ObjectApprovers` instance, the principal
is only required for providing meaningful log messages.

This commit encodes that at the type system by storing the principal
as a string as opposed to a `Principal` object.

Review: https://reviews.apache.org/r/68360/

abudnik/mesos,jpeach/mesos,abudnik/mesos,chhsia0/mesos,kaysoky/mesos,gsantovena/mesos,dforsyth/mesos,dforsyth/mesos,gsantovena/mesos,dforsyth/mesos,andschwa/mesos,kaysoky/mesos,Gilbert88/mesos,Gilbert88/mesos,andschwa/mesos,abudnik/mesos,reneploetz/mesos,dforsyth/mesos,Gilbert88/mesos,kaysoky/mesos,abudnik/mesos,andschwa/mesos,reneploetz/mesos,abudnik/mesos,abudnik/mesos,chhsia0/mesos,jpeach/mesos,chhsia0/mesos,reneploetz/mesos,andschwa/mesos,jpeach/mesos,gsantovena/mesos,gsantovena/mesos,chhsia0/mesos,reneploetz/mesos,dforsyth/mesos,kaysoky/mesos,Gilbert88/mesos,chhsia0/mesos,jpeach/mesos,kaysoky/mesos,jpeach/mesos,dforsyth/mesos,chhsia0/mesos,Gilbert88/mesos,andschwa/mesos,dforsyth/mesos,abudnik/mesos,reneploetz/mesos,chhsia0/mesos,kaysoky/mesos,gsantovena/mesos,gsantovena/mesos,jpeach/mesos,reneploetz/mesos,andschwa/mesos,reneploetz/mesos,jpeach/mesos,kaysoky/mesos,Gilbert88/mesos,Gilbert88/mesos,andschwa/mesos,gsantovena/mesos