refactor: Clean up CTxMemPool initializer list

Shorten the CTxMemPool initializer list using default initialization
for members that dont depend on the constuctor parameters.

Author: ellemouton

src/txmempool.cpp

@@ -332,7 +332,7 @@ void CTxMemPoolEntry::UpdateAncestorState(int64_t modifySize, CAmount modifyFee,
 }
 
 CTxMemPool::CTxMemPool(CBlockPolicyEstimator* estimator, int check_ratio)
-    : m_check_ratio(check_ratio), nTransactionsUpdated(0), minerPolicyEstimator(estimator), m_epoch(0), m_has_epoch_guard(false)
+    : m_check_ratio(check_ratio), minerPolicyEstimator(estimator)
 {
     _clear(); //lock free clear
 }

src/txmempool.h

@@ -489,7 +489,7 @@ class CTxMemPool
 {
 private:
     const int m_check_ratio; //!< Value n means that 1 times in n we check.
-    std::atomic<unsigned int> nTransactionsUpdated; //!< Used by getblocktemplate to trigger CreateNewBlock() invocation
+    std::atomic<unsigned int> nTransactionsUpdated{0}; //!< Used by getblocktemplate to trigger CreateNewBlock() invocation
     CBlockPolicyEstimator* minerPolicyEstimator;
 
     uint64_t totalTxSize;      //!< sum of all mempool tx's virtual sizes. Differs from serialized tx size since witness data is discounted. Defined in BIP 141.
@@ -498,8 +498,8 @@ class CTxMemPool
     mutable int64_t lastRollingFeeUpdate;
     mutable bool blockSinceLastRollingFeeBump;
     mutable double rollingMinimumFeeRate; //!< minimum fee to get into the pool, decreases exponentially
-    mutable uint64_t m_epoch;
-    mutable bool m_has_epoch_guard;
+    mutable uint64_t m_epoch{0};
+    mutable bool m_has_epoch_guard{false};
 
     // In-memory counter for external mempool tracking purposes.
     // This number is incremented once every time a transaction