Add support to store time offsets in cc_keyring

Add support to store time offsets in cc_keyring

The code follows the same model used for the memory ccache type.  Time
offsets are stored in each credential cache in a special key just like
the principal name.  Legacy session caches do not store timestamps as
legacy code would fail when iterating over the new offset key.

[ghudson@mit.edu: minor formatting changes; note legacy session
exception in commit message]

ticket: 7768 (new)
target_version: 1.12
tags: pullup