miner: temporarily disable softfork voting by default.

Several mainnet miners have been mistakenly voting on the RSA hardening
soft-fork. This is a result of pool software creating the template internally,
a la bstratum (i.e. not via getwork or getblocktemplate).

Until miner code either accounts for this or switches to GBT completely, we
should take measures to avoid this.

lib/mining/miner.js

@@ -467,7 +467,7 @@ class MinerOptions {
     this.chain = null;
     this.mempool = null;
 
-    this.version = -1;
+    this.version = 0;
     this.addresses = [];
     this.coinbaseFlags = Buffer.from(`mined by ${pkg.name}`, 'ascii');
     this.preverify = false;