Disable pegin validation to workaround mainchain sync delay (#324)

- elementsd will mark liquid blocks as invalid, if they contain pegins
that reference mainchain blocks that haven't synced
- elementsd does not (yet) recover by itself, see

ElementsProject/elements#891

- when elementsd can automatically revalidate blocks, we
should revert this commit

Co-authored-by: Phil McLean <phil@blockstream.com>

contrib/liquid-mainnet-explorer.conf.in

@@ -2,6 +2,7 @@ server=1
 peerbloomfilters=0
 enforcenodebloom=1
 disablewallet=1
+validatepegin=0
 mainchainrpccookiefile=/data/bitcoin/.cookie
 listenonion=1
 listen=0