Bug# 30907: We don't rely on *first_value to be 0 when checking whether
get_auto_increment() has been invoked for the first time in a multi-row
INSERT. We instead use trx_t::n_autoinc_rows. Initialize trx::n_autoinc_rows
inside ha_innobase::start_stmt() too.

Bug# 30888: While adding code for the low level read of the AUTOINC value
from the index, the case for MEDIUM ints which are 3 bytes was missed
triggering an assertion.