The "MEMORY" ccache is shared in a process.
If a client uses it to make parallel connections,
the same ccache may be used for several clients with distinct
credentials.
To prevent such sharing we create a unique, dedicated ccache when
necessary with krb5_cc_new_unique.
We should destroy the ccaches we created, to avoid leaks.
The struct KRB_CREDENTIALS is extended to express the ccache ownership.
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