shared: Convert direct/backend to BuxtonHashmap

Signed-off-by: Ikey Doherty <michael.i.doherty@intel.com>
libbuxton: Address long standing leak for direct client

We now ensure that we always clean out any cached BuxtonKey's
that have been initialised via the direct client usage in both
the close and library exit stages.

Signed-off-by: Ikey Doherty <michael.i.doherty@intel.com>