page_update_max_trx_id() when buffering op == IBUF_OP_DELETE.  This
addresses Issue #319, an assertion failure because trx_id is zero in
page_update_max_trx_id() when attempting to buffer a purge operation.
(This was a harmless debug assertion failure, because
page_update_max_trx_id() would never update to zero.)

For the purge transaction, trx->id is always zero.  The purge
transaction is the only caller of IBUF_OP_DELETE, and PAGE_MAX_TRX_ID
is really about existing records.  Therefore, there is no need to
update the PAGE_MAX_TRX_ID when purging records.