Improved signature lookup

[notabene2]

Caching the part of the stream which potentially contains the signature improves the performance in case of streams where seeking backwards is costly. If the underlying stream is for example an Azure blob stream which pretends to contain a zip file but actually does not then the lookup took several minutes to figure this out.

I certify that I own, and have sufficient rights to contribute, all source code and related material intended to be compiled or integrated with the source code for the SharpZipLib open source product (the "Contribution"). My Contribution is licensed under the MIT License.