Defined some time-conversion constants.

Change-Id: Iee22eb6cf93fe457017d3de532560b88e7c16b5e
Added Ybuffer_append_format for appending a formatted string

Change-Id: Ie28cb5b379939c11ae5697b6832013242b693ba0
Implement a "push back" API to re-inject buffer into a readable Ychannel.

Also fix bug when requesting reading more than 16kB from Ychannel. Direct I/O
from Ychannel are performed as chunked of 16kB. Pointer to memory to read
buffer into is not advanced on a successful read of a 16kB chunk, causing
attempts to read such large chunks to not populate the target buffer
correctly while still reporting success.

Change-Id: Idd59ec9c758cf2a517c214c3d552b9cd675fbafa