make `wait_for_replication` more robust
make `wait_for_replication` more robust

we discovered that direct string comparision of xlog locations can give
invalid results in some situations, where second part of the xlog
location value "wraps" from 7 hex characters to 8

Change-Id: I62215e5fcc3f9655643aab9fc99f55ddbb37e8ac
Reviewed-on: https://gerrit.instructure.com/150385
Tested-by: Jenkins
Reviewed-by: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
QA-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms