PFSELFTEST improved to test the approximation error.
PFCOUNT: unshare the object when cached cardinality is modified.
PFGETREG added for testing purposes.

The new command allows to get a dump of the registers stored
into an HyperLogLog data structure for testing / debugging purposes.
Remove HyperLogLog type checking duplicated code.
Make hll-gnuplot-graph.rb callable from cli.
Fix PFADD infinite loop.

We need to guarantee that the last bit is 1, otherwise an element may
hash to just zeroes with probability 1/(2^64) and trigger an infinite
loop.

See issue #1657.
Return "WRONGTYPE" error on PF* type mismatch.
Initial HyperLogLog tests.
Changed HyperLogLog hash seed to a non-zero value.

Using a seed of zero has the side effect of having the empty string
hashing to what is a very special case in the context of HyperLogLog: a
very long run of zeroes.

This did not influenced the correctness of the result with 16k registers
because of the harmonic mean, but still it is inconvenient that a so
obvious value maps to a so special hash.

The seed 0xadc83b19 is used instead, which is the first 64 bits of the
SHA1 of the empty string.

Reference: issue #1657.
tryObjectEncoding() refactoring.

We also avoid to re-create an object that is already in EMBSTR encoding.
PFCOUNT: always unshare/decode the object.

This will be a non-op most of the times since the object will be
unshared / decoded, however it is more technically correct to start this
way since the object may be decoded even in the read-only code path.
ZRANGEBYLEX and ZREVRANGEBYLEX implementation.
Add casting to match printf format.

adjustOpenFilesLimit() and clusterUpdateSlotsWithConfig() that were
assuming uint64_t is the same as unsigned long long, which is true
probably for all the systems out there that we target, but still GCC
emitted a warning since technically they are two different types.
HyperLogLog sparse representation description and macros.
HyperLogLog sparse representation slightly modified.

After running a few simulations with different alternative encodings,
it was found that the VAL opcode performs better using 5 bits for the
value and 2 bits for the run length, at least for cardinalities in the
range of interest.
HyperLogLog refactoring to support different encodings.

Metadata are now placed at the start of the representation as an header.
There is a proper structure to access the representation.
Still work to do in order to truly abstract the implementation from the
representation, commands still work assuming dense representation.
hllAdd() refactored into two functions.

Also dense representation access macro renamed accordingly.
hllCount() refactored to support multiple representations.
HyperLogLog sparse representation initial implementation.

Code never tested, but the basic layout is shaped in this commit.
Also missing:

1) Sparse -> Dense conversion function.
2) New HLL object creation using the sparse representation.
3) Implementation of PFMERGE for the sparse representation.
HyperLogLog sparse to dense conversion function.