Better defaults for the Pipe (dotnet/corefx#35939)

Better defaults for the Pipe (dotnet/corefx#35939)

- Use a 4K buffer instead of 2K (4K is a very common buffer size and usually maps to system page size)
- Use a stack for the buffer segment pool and allow pooling more than 16 segments for large writes

Commit migrated from https://github.com/dotnet/corefx/commit/b49b512a6288edd96d4fbb68bf23a4bd2f831545