Bump minrelaytxfee default

To bridge the time until a dynamic method for determining this fee is
merged.

This is especially aimed at the stable releases (0.10, 0.11) because
full mempool limiting, as will be in 0.12, is too invasive and risky to
backport.

src/main.cpp

@@ -75,7 +75,7 @@ uint64_t nPruneTarget = 0;
 bool fAlerts = DEFAULT_ALERTS;
 
 /** Fees smaller than this (in satoshi) are considered zero fee (for relaying and mining) */
-CFeeRate minRelayTxFee = CFeeRate(1000);
+CFeeRate minRelayTxFee = CFeeRate(5000);
 
 CTxMemPool mempool(::minRelayTxFee);