Before this PR, we use sdsMakeRoomFor() to expand the size of hyperloglog
string (sparse representation). And because sdsMakeRoomFor() uses a greedy
strategy (allocate about twice what we need), the memory we allocated for the
hyperloglog may be more than `server.hll_sparse_max_bytes` bytes.
The memory more than` server.hll_sparse_max_bytes` will be wasted.
In this pull request, tone down the greediness of the allocation growth, and also
make sure it'll never request more than `server.hll_sparse_max_bytes`.
This could in theory mean the size of the hyperloglog string is insufficient for the
increment we need, should be ok since in this case we promote the hyperloglog
to dense representation, an assertion was added to make sure.
This PR also add some tests and fixes some typo and indentation issues.
f8ac5a6503