MINOR: htx: Add helper functions to xfer a message to smaller or larger one

MINOR: htx: Add helper functions to xfer a message to smaller or larger one

htx_move_to_small_buffer()/htx_move_to_large_buffer() and
htx_copy_to_small_buffer()/htx_copy_to_large_buffer() functions can now be
used to move or copy blocks from a default buffer to a small or large
buffer. The destination buffer is allocated and then each blocks are
transferred into it.

These funtions relies in htx_xfer() function.