use DEBUG DIGEST in the test instead of a function that was doing a similar work, but in a much slower and buggy way

DEBUG DIGEST implemented, in order to improve the ability to test persistence and replication consistency

very strong speedup in saving time performance when there are many integers in the dataset. Instead of decoding the object before to pass them to the rdbSaveObject layer we check asap if the object is integer encoded and can be written on disk as an integer.

added overflow check in the double -> long long conversion trick to avoid integer overflows. I think this was not needed in practical terms, but it is safer

If a float can be casted to a long long without rounding loss, we can use the integer conversion function to write the score on disk. This is a seriuous speedup

fixed a problem leading to crashes, as keys can't be currently specially encoded, so we can't encode integers at object loading time... For now this can be fixed passing a few flags, or later can be fixed allowing encoded keys as well

long long to string conversion speedup applied in other places as well. Still the code has bugs, fixing right now...

added specialized function to compare string objects for perfect match that is optimized for this task

while loading the rdb file don't add the key to the dictionary at all if it's already expired, instead of removing it just after the insertion.

swap objects out directly while loading an RDB file if we detect we can't stay in the vm max memory limits anyway