libafs: retry retriable RPCs instead of abandoning

libafs: retry retriable RPCs instead of abandoning

if we get e.g. an idle dead error we should retry
retriable actions, namely data stores. in order
for writing files to work correctly given how
the writeback code is structured it's important that
this not interfere with analyze's shouldRetry decision
on those RPCs

Change-Id: Ie091a72b924c8414ec66c377fa13da14575a69cf
Reviewed-on: http://gerrit.openafs.org/6749
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>