refactor: Clean up CTxMemPool initializer list

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

This is a backport of [[bitcoin/bitcoin#20222 | core#20222]] [3/3]
bitcoin/bitcoin@f15e780

Depends on D10842

Test Plan:
`ninja all check-all`

eviewers:

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10843

src/txmempool.cpp

@@ -399,9 +399,7 @@ void CTxMemPoolEntry::UpdateAncestorState(int64_t modifySize, Amount modifyFee,
     assert(int(nSigOpCountWithAncestors) >= 0);
 }
 
-CTxMemPool::CTxMemPool(int check_ratio)
-    : m_check_ratio(check_ratio), nTransactionsUpdated(0), m_epoch(0),
-      m_has_epoch_guard(false) {
+CTxMemPool::CTxMemPool(int check_ratio) : m_check_ratio(check_ratio) {
     // lock free clear
     _clear();
 }

src/txmempool.h

@@ -497,7 +497,7 @@ class CTxMemPool {
     //! Value n means that 1 times in n we check.
     const int m_check_ratio;
     //! Used by getblocktemplate to trigger CreateNewBlock() invocation
-    std::atomic<uint32_t> nTransactionsUpdated;
+    std::atomic<uint32_t> nTransactionsUpdated{0};
 
     //! sum of all mempool tx's sizes.
     uint64_t totalTxSize;
@@ -509,8 +509,8 @@ class CTxMemPool {
     mutable bool blockSinceLastRollingFeeBump;
     //! minimum fee to get into the pool, decreases exponentially
     mutable double rollingMinimumFeeRate;
-    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