## Issue:
When a dict has a long chain or the length of the chain is longer than
the number of samples, we will never be able to sample the elements
at the end of the chain using dictGetSomeKeys().
This could mean that SRANDMEMBER can be hang in and endless loop.
The most severe case, is the pathological case of when someone uses SCAN+DEL
or SSCAN+SREM creating an unevenly distributed dict.
This was amplified by the recent change in #11692 which prevented a
down-sizing rehashing while there is a fork.
## Solution
1. Before, we will stop sampling when we reach the maximum number
of samples, even if there is more data after the current chain.
Now when we reach the maximum we use the Reservoir Sampling
algorithm to fairly sample the end of the chain that cannot be sampled
2. Fix the rehashing code, so that the same as it allows rehashing for up-sizing
during fork when the ratio is extreme, it will allow it for down-sizing as well.
Issue was introduced (or became more severe) by #11692
Co-authored-by: Oran Agra <oran@redislabs.com>
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