Since the forked process allocates the memory from the system heap,
it must verify if the address is in the system heap or in the
redis heap before freeing it.
Changed dictRehash to NOOP when called by the forked process to avoid
extra processing that is not required when the forked process is
saving the dataset.

[Fix] Windows portability: explicit type casting.
[Cleanup] Code refactoring. Comments. Changed functions prefix to match
the functions type.

Conflicts:
	src/anet.c
	src/aof.c
	src/bitops.c
	src/config.c
	src/db.c
	src/debug.c
	src/dict.c
	src/migrate.c
	src/object.c
	src/redis.c
	src/redis.h
	src/sentinel.c
	src/t_list.c
	src/t_set.c
	src/t_zset.c
	src/util.c
	tests/instances.tcl

rehashidx is always positive in the two code paths, since the only
negative value it could have is -1 when there is no rehashing in
progress, and the condition is explicitly checked.

Fixed by @oranagra, thank you.

Avoid code repetition introduced with PR #2367, also fixes the return
value to always return 0 if there is nothing more to rehash.

This is very similar to the optimization applied to dictGetRandomKeys,
but applied to the single key variant.

Related to issue #2306.

Related to issue #2306.

We use the invariant that the original table ht[0] is never populated up
to the index before the current rehashing index.

Related to issue #2306.

Related to issue #2306.

Related to issue #2306.

Related to issue #2306.

Some language in the comment was difficult
to understand, so this commit: clarifies wording, removes
unnecessary words, and relocates some dependent clauses
closer to what they actually describe.

I also tried to break up longer chains of thought
(if X, then Y, and Q, and also F, so obviously M)
into more manageable chunks for ease of understanding.

Closes #1351

Also related to #1929.

This closes issue #1929, the other part was fixed in the context of issue

Closes #878

This new function is useful to get a number of random entries from an
hash table when we just need to do some sampling without particularly
good distribution.

It just jumps at a random place of the hash table and returns the first
N items encountered by scanning linearly.

The main usefulness of this function is to speedup Redis internal
sampling of the key space, for example for key eviction or expiry.

Redis hash table implementation has many non-blocking features like
incremental rehashing, however while deleting a large hash table there
was no way to have a callback called to do some incremental work.

This commit adds this support, as an optiona callback argument to
dictEmpty() that is currently called at a fixed interval (one time every
65k deletions).

The irrelevant bits shouldn't be masked to 1. This can result in slots being
skipped when the hash table is resized between calls to the iterator.

This reverts commit 6253180a.

Also a warning was suppressed by including unistd.h in redisassert.h
(needed for _exit()).

The previous hashing used the trivial algorithm of xoring the integers
together. This is not optimal as it is very likely that different
hash table setups will hash the same, for instance an hash table at the
start of the rehashing process, and at the end, will have the same
fingerprint.

Now we hash N integers in a smarter way, by summing every integer to the
previous hash, and taking the integer hashing again (see the code for
further details). This way it is a lot less likely that we get a
collision. Moreover this way of hashing explicitly protects from the
same set of integers in a different order to hash to the same number.

This commit is related to issue #1240.