CLUSTER RESET: Flush dataset if node is a slave.

For non-empty masters, CLUSTER RESET is denied, and the user requires to
start to reset a node by explicitly clearing it with FLUSHALL.
However CLUSTER RESET when executed with slaves don't have this
restrictions since data is just a replica of the master, and with
read-only slaves it is also not possible to remove the data set. However
the node was turned from slave to master after a reset, without touching
the old slave data. This is 99.99% of times not appropriate and forces
full resets to follow this path to work with both slave and master
nodes:

    FLUSHALL
    CLUSTER RESET HARD
    FLUSHALL

Since we need the first flushall for masters, and the second for slaves.

This commit changes the behavior so that CLUSTER RESET removes the data set
of a slave node during a reset, in the moment it gets turned into a master,
so the new pattern is simply:

    FLUSHALL (that may fail for slaves)
    CLUSTER RESET
Add double field in dict.c entry value union.
ZUNIONSTORE reimplemented for speed.

The user @kjmph provided excellent ideas to improve speed of ZUNIONSTORE
(in certain cases by many order of magnitude), together with an
implementation of the ideas.

While the ideas were sounding, the implementation could be improved both
in terms of speed and clearness, so that's my attempt at reimplementing
the speedup proposed, trying to improve by directly using just a
dictionary with an embedded score inside, and reusing the single-pass
aggregate + order-later approach.

Note that you can't apply this commit without applying the previous
commit in this branch that adds a double in the dictEntry value union.

Issue #1786.
Remove useless var and check in zunionInterGenericCommand().
Test: check sorted set elements order after union.

This is not a regression but issue #1786 showed the need for this test.