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- Patch by David Carlier

Also, updateLease4 and updateLease6 are implemented.

This change applies to the Memfile backend and is intended to significantly
improve its performance.

Also make the hardware address to string function common to both
database modules.

As the address is th eprimary key of the table, there can only ever
be at most one lease in the database with a given address.  Additional
selection by subnet ID is not needed.

This reflects the fact that at the moment, the data is stored in
memory and not in a backing file.

The message when the DHCPv6 database backend is started now includes
both the type and the name of the database.