[PLAT-4658] Added handling for missing market data in snapshots which is more consistent with live handling. Missing ids are treated by the FixedMarketDataAvailabilityProvider in the same way as available market data; i.e. they are reported to the engine as available. When the graph comes to execute, they will be returned as missing from the market data sourcing function. (Extra handling in the MarketDataAliasingFunction was required in order to avoid an evaluation error here).

Other cleanups include removing unused methods (and corresponding tests) from FixedMarketDataAvailabilityProvider.