vppinfra: fix corner-cases in bihash lookup

vppinfra: fix corner-cases in bihash lookup

In a case where one pounds on a single kvp in a KVP_AT_BUCKET_LEVEL
table, the code would sporadically return a transitional value (junk)
from a half-deleted kvp. At most, 64-bits worth of the kvp will be
written atomically, so using memset(...) to smear 0xFF's across a kvp
to free it left a lot to be desired.

Performance impact: very mild positive, thanks to FC for doing a
multi-thread host stack perf/scale test.

Added an ASSERT to catch attempts to add a (key,value) pair which
contains the magic "free kvp" value.

Type: fix

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I6a1aa8a2c30bc70bec4b696ce7b17c2839927065