Optimize hgetall for large hash table

[sunxiaoguang]

Although hgetall is inherently slow and should be used with caution, but there are cases it's necessary to fetch everything at once. This patch tries to optimize this code path.

[soloestoy]

@sunxiaoguang It seems like an optimization about the special encoding OBJ_ENCODING_HT, can u give an accurate percentage about the optimization?

[sunxiaoguang]

@soloestoy Here is the comment I posted on the original pull request to vanilla version. And I copied it here for your convenience as well.
A hash gets converted to dict encoding if it's getting, depending on the configuration, more than 512 entries by default. Iterating such a large hash table using the iteration api takes quite some cpu cycles and generated more cache misses then necessary. This merge request uses a dedicated iterate all function to traverse the whole dict to generate hgetall, hkeys and hvals response, which can reduce latency by 1 ~ 5% depends on size of the dict.

The code to generate test data and benchmark the difference can be found here. Please use the 'optimize_hgetall_unstable_comparaison' branch which contains commands for both the original implementation and new implementation for reference.

Some test runs of the benchmark program on a E5-2670 v3 server demonstrate for a 16000 fields hash table (the test h5) the new way can save couple hundreds of microseconds on average for a hgetall call. Running the same test on servers with less cache can observe some more improvements as the iterator way access data in a more scattered way therefore renders cache less efficient.

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