MINOR: quic: rename confusing qc_purge_txbuf() to qc_flush_txbuf()

MINOR: quic: rename confusing qc_purge_txbuf() to qc_flush_txbuf()

Function qc_purge_txbuf() naming is confusing as it may imply that
prepared data are simply discarded. In fact, the objective of this
function is to first send all of its content before clearing it.

This names also conflicts with qc_purge_tx_buf() (note the extra
underscore) which reset the buffer. Thus, renaming the first one is
better to prevent any confusion.