HADOOP-11161. Expose close method in KeyProvider to give clients of Provider implementations a hook to release resources. Contribued by Arun Suresh.
(cherry picked from commit d9556e873ef4d3e68c4f0c991f856d1faa747f07)
(cherry picked from commit 3a2565c7be80cf6e9cdfec0f5460ed8ed2252768)