Closes #1969. Default fee now sufficient for large shielded tx.

Sets default min relay tx fee to 100, which is used to initialize
nSatoshisPerK for min fee calculation.  The default z_sendmany
fee of 10000 is now sufficient to cover a tx of size 100000 bytes,
so txs will not be treated as "free" and hit the free rate limiter.

src/main.h

@@ -61,7 +61,7 @@ static const unsigned int MAX_P2SH_SIGOPS = 15;
 /** The maximum number of sigops we're willing to relay/mine in a single tx */
 static const unsigned int MAX_STANDARD_TX_SIGOPS = MAX_BLOCK_SIGOPS/5;
 /** Default for -minrelaytxfee, minimum relay fee for transactions */
-static const unsigned int DEFAULT_MIN_RELAY_TX_FEE = 1000;
+static const unsigned int DEFAULT_MIN_RELAY_TX_FEE = 100;
 /** Default for -maxorphantx, maximum number of orphan transactions kept in memory */
 static const unsigned int DEFAULT_MAX_ORPHAN_TRANSACTIONS = 100;
 /** The maximum size of a blk?????.dat file (since 0.8) */