discoverd/server: Fix improper SetLeader write mutex
discoverd/server: Fix improper SetLeader write mutex

Writes should not use the read side of the mutex.

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>

flynn/flynn,supermario/flynn,kgrz/flynn,rikur/flynn,jzila/flynn,jzila/flynn,jzila/flynn,whouses/flynn,justintung/flynn,flynn/flynn,pkdevbox/flynn,shads196770/flynn,flynn/flynn,Brandywine2161/flynn,justintung/flynn,shads196770/flynn,schatt/flynn,ozum/flynn,rikur/flynn,Brandywine2161/flynn,supermario/flynn,justintung/flynn,shads196770/flynn,whouses/flynn,supermario/flynn,schatt/flynn,pkdevbox/flynn,TribeMedia/flynn,whouses/flynn,pkdevbox/flynn,lmars/flynn,jzila/flynn,benbjohnson/flynn,ozum/flynn,TribeMedia/flynn,shads196770/flynn,pkdevbox/flynn,jzila/flynn,Brandywine2161/flynn,kgrz/flynn,rikur/flynn,TribeMedia/flynn,Brandywine2161/flynn,josephglanville/flynn,justintung/flynn,TribeMedia/flynn,rikur/flynn,whouses/flynn,whouses/flynn,philiplb/flynn,shads196770/flynn,rikur/flynn,philiplb/flynn,schatt/flynn,clifff/flynn,benbjohnson/flynn,kgrz/flynn,rikur/flynn,justintung/flynn,lmars/flynn,kgrz/flynn,lmars/flynn,supermario/flynn,ozum/flynn,ozum/flynn,flynn/flynn,justintung/flynn,clifff/flynn,supermario/flynn,Brandywine2161/flynn,benbjohnson/flynn,benbjohnson/flynn,philiplb/flynn,schatt/flynn,pkdevbox/flynn,lmars/flynn,ozum/flynn,clifff/flynn,clifff/flynn,josephglanville/flynn,ozum/flynn,whouses/flynn,TribeMedia/flynn,philiplb/flynn,pkdevbox/flynn,flynn/flynn,benbjohnson/flynn,philiplb/flynn,schatt/flynn,TribeMedia/flynn,kgrz/flynn,lmars/flynn,schatt/flynn,shads196770/flynn,benbjohnson/flynn,josephglanville/flynn,josephglanville/flynn,clifff/flynn,clifff/flynn,Brandywine2161/flynn,josephglanville/flynn,jzila/flynn,kgrz/flynn,josephglanville/flynn,supermario/flynn