storage_proxy: drop overzealous use of __int128_t in recently-modified-no-read-repair logic
storage_proxy: drop overzealous use of __int128_t in recently-modified-no-read-repair logic

Clang's std::abs() doesn't support __int128_t, so use __int64_t instead.  With
this change, it's possible that a read repair 252,700 years after a write
will be interpreted as a recent write and the read repair will incorrectly
be skipped; hopefully by that time __int128_t will be standardized.

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