Disable snapshot refresh feature when snap_refresh_nanos is 0 (#5724)

Disable snapshot refresh feature when snap_refresh_nanos is 0 (#5724)

Summary:
The comments of snap_refresh_nanos advertise that the snapshot refresh feature will be disabled when the option is set to 0. This contract is however not honored in the code: https://github.com/facebook/rocksdb/pull/5278
The patch fixes that and also adds an assert to ensure that the feature is not used when the option  is zero.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5724

Differential Revision: D16918185

Pulled By: maysamyabandeh

fbshipit-source-id: fec167287df7d85093e087fc39c0eb243e3bbd7e