Add OncePer.Get()

Allow functions to get the result associated with a OncePer key without
also specifiying a function.  Panics if the key has not already been
set.  Also replace the open-coded concurrent map implementation with
the new sync.Map.

Test: m checkbuild
Change-Id: I814fdb1ffffaee8398dc877af146e29638c8a6a8