Chunksize should be configurable when using copy_from

When using the copy_from option, readers and writers can have different speeds to respectively read and write.
A reader can timeout if the connection has been opened for too long without getting any heartbeat.
This is happening if the writer is slower than the reader. A reader will have to wait for the writer to process all the bits sent before
getting the next chunk. If this time to process the bits is too long the reader will timeout.
Consequently, the writer should be able to control the size of the chunks that needs to be read in order to be able to write fast enough.