HBASE-20391 close out stale or finished PRs on github.
HBASE-20391 close out stale or finished PRs on github.

* closes #51 - > 1 month since notification
* closes #61 - HBASE-18928 has already closed
* closes #62 - HBASE-18929 has already closed
* closes #67 - HBASE-19386 has already closed
* closes #68 - HBASE-19387 has already closed

Also adds a section to the committer guide in the reference guide about closing PRs.

Signed-off-by: Mike Drob <7d5db5eeae20c13a709f5640b51f5c63dc0a2d15@apache.org>

francisliu/hbase,ndimiduk/hbase,ChinmaySKulkarni/hbase,Apache9/hbase,ndimiduk/hbase,ultratendency/hbase,Eshcar/hbase,Eshcar/hbase,apurtell/hbase,ndimiduk/hbase,ultratendency/hbase,bijugs/hbase,Apache9/hbase,mahak/hbase,ndimiduk/hbase,mahak/hbase,bijugs/hbase,apurtell/hbase,apurtell/hbase,Eshcar/hbase,Apache9/hbase,Apache9/hbase,ChinmaySKulkarni/hbase,mahak/hbase,francisliu/hbase,mahak/hbase,Apache9/hbase,ndimiduk/hbase,apurtell/hbase,francisliu/hbase,ndimiduk/hbase,ultratendency/hbase,ChinmaySKulkarni/hbase,ultratendency/hbase,apurtell/hbase,ChinmaySKulkarni/hbase,francisliu/hbase,ultratendency/hbase,ultratendency/hbase,bijugs/hbase,mahak/hbase,ndimiduk/hbase,ChinmaySKulkarni/hbase,ChinmaySKulkarni/hbase,bijugs/hbase,francisliu/hbase,Eshcar/hbase,Eshcar/hbase,francisliu/hbase,ndimiduk/hbase,mahak/hbase,bijugs/hbase,ultratendency/hbase,Apache9/hbase,Apache9/hbase,francisliu/hbase,mahak/hbase,Apache9/hbase,Eshcar/hbase,Eshcar/hbase,apurtell/hbase,ChinmaySKulkarni/hbase,Eshcar/hbase,ChinmaySKulkarni/hbase,apurtell/hbase,mahak/hbase,Eshcar/hbase,ultratendency/hbase,ultratendency/hbase,ChinmaySKulkarni/hbase,bijugs/hbase,bijugs/hbase,Apache9/hbase,francisliu/hbase,bijugs/hbase,ultratendency/hbase,apurtell/hbase,francisliu/hbase,apurtell/hbase,bijugs/hbase,francisliu/hbase,ndimiduk/hbase,Apache9/hbase,mahak/hbase,Eshcar/hbase,ndimiduk/hbase,mahak/hbase,ChinmaySKulkarni/hbase,apurtell/hbase,bijugs/hbase