[4294] Changed Memfile stats recount to use existing per-address index

[4294] Changed Memfile stats recount to use existing per-address index

src/lib/dhcpsrv/memfile_lease_mgr.cc
    MemfileLeaseStatsQuery4::start()
    MemfileLeaseStatsQuery6::start() - both now rely on per-address index,
    rather than adding per subnet_id index.

src/lib/dhcpsrv/memfile_lease_storage.h
    Removed per subnet_id indexes