Fall back to credential refresh on EDEADLK

Fixes https://github.com/google/oauth2client/issues/335

This change fixes a bug where multiple threads and/or processes
using multistore_file to access the same backing store could
raise IOError errno.EDEADLK to the calling application. Since
EDEADLK is a possibility with concurrent access, this change
instead causes a fallback to read only mode and refresh
credentials if necessary.