[neo-project/neo] Prioritize verification of higher fee TXs and do not starve RpcServer w/ Large mempool (#356)

In MSDN, I saw a description like this:

> `ReaderWriterLock` works best where most accesses are reads, while writes are infrequent and of short duration.

But most of the operations in «Cache.cs» are writes. So I don’t think this will improve performance.

https://docs.microsoft.com/en-us/dotnet/api/system.threading.readerwriterlock