Initial support for parallel Soroban phase XDR.

I've tried to minimize the scope of the changes; specifically this doesn't contain any actual logic for the parallel execution (such as data dependency validation and building parallel stages). However, there is still some refactoring that needed to happen in order to support new, more complex tx sets.

src/herder/HerderImpl.cpp

@@ -1350,7 +1350,7 @@ HerderImpl::triggerNextLedger(uint32_t ledgerSeqToTrigger,
     // our first choice for this round's set is all the tx we have collected
     // during last few ledger closes
     auto const& lcl = mLedgerManager.getLastClosedLedgerHeader();
-    TxSetPhaseTransactions txPhases;
+    PerPhaseTransactionList txPhases;
     txPhases.emplace_back(mTransactionQueue.getTransactions(lcl.header));
 
     if (protocolVersionStartsFrom(lcl.header.ledgerVersion,
@@ -1415,7 +1415,7 @@ HerderImpl::triggerNextLedger(uint32_t ledgerSeqToTrigger,
     upperBoundCloseTimeOffset = nextCloseTime - lcl.header.scpValue.closeTime;
     lowerBoundCloseTimeOffset = upperBoundCloseTimeOffset;
 
-    TxSetPhaseTransactions invalidTxPhases;
+    PerPhaseTransactionList invalidTxPhases;
     invalidTxPhases.resize(txPhases.size());
 
     auto [proposedSet, applicableProposedSet] =

src/herder/TransactionQueue.cpp

@@ -938,11 +938,11 @@ TransactionQueue::isBanned(Hash const& hash) const
         });
 }
 
-TxSetTransactions
+TxFrameList
 TransactionQueue::getTransactions(LedgerHeader const& lcl) const
 {
     ZoneScoped;
-    TxSetTransactions txs;
+    TxFrameList txs;
 
     uint32_t const nextLedgerSeq = lcl.ledgerSeq + 1;
     int64_t const startingSeq = getStartingSequenceNumber(nextLedgerSeq);

src/herder/TransactionQueue.h

@@ -143,7 +143,7 @@ class TransactionQueue
 
     bool isBanned(Hash const& hash) const;
     TransactionFrameBaseConstPtr getTx(Hash const& hash) const;
-    TxSetTransactions getTransactions(LedgerHeader const& lcl) const;
+    TxFrameList getTransactions(LedgerHeader const& lcl) const;
     bool sourceAccountPending(AccountID const& accountID) const;
 
     virtual size_t getMaxQueueSizeOps() const = 0;