cleanup and merge orderbook not crossed invariant

[MonsieurNicolas]

#2210

[MonsieurNicolas]

this implements one of #2884

[rokopt]

A couple notes/questions:

• In the current OrderBookIsNotCrossed invariant #2210 , the new invariant is compiled in and enabled when and only when we compile for fuzzing (FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION). Should we instead do this as we did the best-offer debugging (Best-offer debugging #2944 ) and compile it in when and only when extrachecks are enabled, and use a new Config parameter to make sure it's only used during stellar-core fuzz and stellar-core test (not, for example, stellar-core run or stellar-core catchup)?
• The invariant as is can fire incorrectly because it doesn't know about passive offers. I think that the right fix is to test if (lowestAsk == highestBid) in checkCrossed() and in that case to fire the assertion only if there is at least one non-passive ask at that price and at least one non-passive bid at that price. But please let me know if that doesn't sound right!

[rokopt]

• In the current OrderBookIsNotCrossed invariant #2210 , the new invariant is compiled in and enabled when and only when we compile for fuzzing (FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION). Should we instead do this as we did the best-offer debugging (Best-offer debugging #2944 ) and compile it in when and only when extrachecks are enabled, and use a new Config parameter to make sure it's only used during stellar-core fuzz and stellar-core test (not, for example, stellar-core run or stellar-core catchup)?

After further thought, I realize that we can't do that with the order-book-not-crossed invariant: unlike other invariants, it maintains state (mOrderBook) from one call to checkOnOperationApply() to another. There's no provision for that in InvariantManager, and in particular it won't work because the order-book invariant won't know if the transaction whose operations had been applied were later rolled back. Consequently, it might think (in its mOrderBook) that there's an offer present which isn't, so it might spuriously believe that the order book is crossed when some new offer appears that would have crossed with that offer if it really still existed.