Resolves:
https://fedorahosted.org/sssd/ticket/2485

Reviewed-by: Michal Židek <mzidek@redhat.com>

struct sss_mc_rec had two hash members (hash1 and hash2) but only one next
member. This was a big problem in case of higher probability of hash collision.

structure sss_mc_rec will have two next members (next1, next2) with this patch.
next1 is related to hash1 and next2 is related to hash1.

Iterating over chains is changed, because we need to choose right next pointer.
Right next pointer will be chosen after comparing record hashes.
This behaviour is wrapped in function sss_mc_next_slot_with_hash.

Adding new record to chain is also changed. The situation is very similar to
iterating. We need to choose right next pointer (next1 or next2).
Right next pointer will be chosen after comparing record hashes.
Adding reference to next slot is wrapped in function
sss_mc_chain_slot_to_record_with_hash

Size of structure sss_mc_rec was increased from 32 bytes to 40 bytes.

Resolves:
https://fedorahosted.org/sssd/ticket/2049

We introduced new way to check integrity of memcache in the
client code. We should use similiar checks in the responder.

Removes off by one error when using macro MC_SIZE_TO_SLOTS
and adds new macro MC_SLOT_WITHIN_BOUNDS.

This patch prevents jumping outside of allocated memory in
case of corrupted slot or name_ptr values. It is not proper
solution, just hotfix until we find out what is the root cause
of ticket https://fedorahosted.org/sssd/ticket/2018

We should never try to invalidate an already invalid record as
internal pointers will not be consistent. Carefully test that the
record really is valid when we are fishing for free space, and
properly invalidate records or return a fatal error if something
goes wrong.
In order to make the code more robust always invalidate the whole
data space on initialization by setting all bits to 1, and make sure
to invalidate the whole last allocated slot by converting rec->len to
the number of slots instead of just the space used.