messages,mon,osd: silence gcc-8 warnings related to memset()

messages,mon,osd: silence gcc-8 warnings related to memset()

this silences warnings like:

warning: ‘void* memset(void*, int, size_t)’ writing to an object of
non-trivial type ‘struct uuid_d’; use assignment instead [-Wcl\
ass-memaccess]

uuid_d only contains boost::uuids::uuid, which is "nil" initialized in
uuid_d's ctor. so we don't need to bother with memset() it with 0.
the same applies to entity_inst_t.

Signed-off-by: Kefu Chai <kchai@redhat.com>