Until now, write isolation policy was hardcoded to always enforcing LWT.
From now on, setting a tag via UpdateTags request or during table
creation will associate a policy with given table.
The tag key is 'system:write_isolation' and its value can be one of:
* 'f' - forbid RMW
* 'a' - always enforce RMW
* 'o' - only RMW writes will go through LWT
* 'u' - unsafe RMW (to be deprecated/eradicated)
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