reset fast-forward map before failing over node
reset fast-forward map before failing over node

Otherwise fast-forward map might contain node that's not in list of
servers which will cause map-to-json code to fail.

Change-Id: Id80c4e570da1eb57d84aa11ede6757112ca0e597
Reviewed-on: http://review.membase.org/5918
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

mschoch/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server,daverigby/ns_server,ceejatec/ns_server,ceejatec/ns_server,vzasade/ns_server,daverigby/ns_server,mschoch/ns_server,membase/ns_server,mschoch/ns_server,ceejatec/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,membase/ns_server,vzasade/ns_server,membase/ns_server,membase/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,membase/ns_server,pavel-blagodov/ns_server,vzasade/ns_server,mschoch/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,vzasade/ns_server,pavel-blagodov/ns_server,daverigby/ns_server,membase/ns_server,daverigby/ns_server,ceejatec/ns_server