Enable payload_index_skip_rocksdb feature flag by default.

I'd like to include this in the 1.14.1 release.

Motivation:

• faster index loading on start:
  • Add in-memory payload indices on mmap storage (numeric, map) #6444
  • Add in-memory payload index on mmap storage (full text) #6495
  • Add in-memory payload index on mmap storage (geo) #6503)
• we're satisfied with the results in our CI benchmarks
• backwards compatible, older versions will just load it as mmap index

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2. Have you formatted your code locally using cargo +nightly fmt --all command prior to submission?
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