fix locking issue with new mempool limiting

Current master crashes on OSX with an exception: "boost: mutex lock failed in pthread_mutex_lock: Invalid argument"

src/txmempool.cpp

@@ -309,7 +309,7 @@ void CTxMemPoolEntry::UpdateState(int64_t modifySize, CAmount modifyFee, int64_t
 CTxMemPool::CTxMemPool(const CFeeRate& _minReasonableRelayFee) :
     nTransactionsUpdated(0)
 {
-    clear();
+    _clear(); //lock free clear
 
     // Sanity checks off by default for performance, because otherwise
     // accepting transactions becomes O(N^2) where N is the number
@@ -546,9 +546,8 @@ void CTxMemPool::removeForBlock(const std::vector<CTransaction>& vtx, unsigned i
     blockSinceLastRollingFeeBump = true;
 }
 
-void CTxMemPool::clear()
+void CTxMemPool::_clear()
 {
-    LOCK(cs);
     mapLinks.clear();
     mapTx.clear();
     mapNextTx.clear();
@@ -560,6 +559,12 @@ void CTxMemPool::clear()
     ++nTransactionsUpdated;
 }
 
+void CTxMemPool::clear()
+{
+    LOCK(cs);
+    _clear();
+}
+
 void CTxMemPool::check(const CCoinsViewCache *pcoins) const
 {
     if (!fSanityCheck)

src/txmempool.h

@@ -375,6 +375,7 @@ public:
     void removeForBlock(const std::vector<CTransaction>& vtx, unsigned int nBlockHeight,
                         std::list<CTransaction>& conflicts, bool fCurrentEstimate = true);
     void clear();
+    void _clear(); //lock free
     void queryHashes(std::vector<uint256>& vtxid);
     void pruneSpent(const uint256& hash, CCoins &coins);
     unsigned int GetTransactionsUpdated() const;