NIFI-5134 Explicitly requesting UGI to relogin before attempting to g…

[jtstorck]

…et a DB connection in HiveConnectionPool

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[jtstorck]

@mattyb149, @markap14, would you please review this PR?

[mattyb149]

+1 LGTM, looks like it will work like a barrier sync, the first one in will relogin if necessary, the rest will no-op, which means there won't be a race condition / threading issue when some of the threads call doAs. Thanks for the improvement! Merging to master