Remove meaningless finalizers from library classes #526

Many library classes feature meaningless finalizers that do nothing at runtime but grow the finalization queue and thus slow down GC and application performance.
This is especially concerning for frequently-used classes like SshCommand and Channel.
The library classes don't need finalizers as they aren't dealing with allocation and deallocation of unmanaged resource handles.
The nuances of a correct IDisposable implementation are neatly summarized here:
https://github.com/sidristij/dotnetbook/blob/master/book/en/LifetimeManagement/2-Disposable.md