Minimize buffer allocations in Stream.CopyTo for seekable streams (dotnet/coreclr...

Minimize buffer allocations in Stream.CopyTo for seekable streams (dotnet/coreclr#4540)

The current implementation of  Stream.CopyTo  allocates a giant, 81920-byte buffer if no  bufferSize  parameter is passed. This is incredibly wasteful if the stream we're copying from can seek, because then we can use the  Length  and  Position  properties to determine how many bytes are left and allocate a buffer of that size.

Commit migrated from https://github.com/dotnet/coreclr/commit/91eb03162ceef78beb33548cacd98105376a2a4c