fix race in RPCClient.Dispatch
fix race in RPCClient.Dispatch

The pending map was being used in multiple goroutines without
synchronization.

Found by adding the race detector to testing code. PR to come shortly.

modulexcite/boulder,kuba/boulder,r0ro/boulder,ZCloud-Firstserver/boulder,jmhodges/boulder,devpaul/boulder,jsha/boulder,kuba/boulder,benileo/boulder,julienschmidt/boulder,KyleChamberlin/boulder,lmcro/boulder,sophie-h/boulder,postfix/boulder,jfrazelle/boulder,mozmark/boulder,tomclegg/boulder,jgillula/boulder,mehmooda/boulder,mommel/boulder,jsha/boulder,lmcro/boulder,mozmark/boulder,ZCloud-Firstserver/boulder,KyleChamberlin/boulder,kuba/boulder,bmw/boulder,jcjones/boulder,mozmark/boulder,jsha/boulder,benileo/boulder,jcjones/boulder,deserted/boulder,lucasallan/boulder,patf/boulder,julienschmidt/boulder,lmcro/boulder,ZCloud-Firstserver/boulder,josephyzhou/boulder,lmcro/boulder,jmhodges/boulder,modulexcite/boulder,sophie-h/boulder,mehmooda/boulder,jfrazelle/boulder,tomclegg/boulder,mommel/boulder,benileo/boulder,devpaul/boulder,bmw/boulder,r0ro/boulder,mehmooda/boulder,jfrazelle/boulder,ZCloud-Firstserver/boulder,jmhodges/boulder,jmhodges/boulder,ibukanov/boulder,patf/boulder,modulexcite/boulder,lucasallan/boulder,josephyzhou/boulder,jcjones/boulder,postfix/boulder,mommel/boulder,mozmark/boulder,r0ro/boulder,tomclegg/boulder,deserted/boulder,julienschmidt/boulder,ibukanov/boulder,tomclegg/boulder,kuba/boulder,devpaul/boulder,deserted/boulder,jgillula/boulder,hlandauf/boulder,mommel/boulder,KyleChamberlin/boulder,mehmooda/boulder,jgillula/boulder,patf/boulder,postfix/boulder,benileo/boulder,hlandauf/boulder,jcjones/boulder,jgillula/boulder,hlandauf/boulder,letsencrypt/boulder,modulexcite/boulder,ibukanov/boulder,josephyzhou/boulder,jfrazelle/boulder,patf/boulder,KyleChamberlin/boulder,josephyzhou/boulder,deserted/boulder,lucasallan/boulder,sophie-h/boulder,devpaul/boulder,postfix/boulder,letsencrypt/boulder,hlandauf/boulder,r0ro/boulder,letsencrypt/boulder,julienschmidt/boulder,bmw/boulder