read_buffer_size set on master
BUG#33413 show binlog events fails if binlog has event size of close
          to max_allowed_packet


The size of Append_block replication event was determined solely by
read_buffer_size whereas the rest of replication code deals with
max_allowed_packet.
When the former parameter was set to larger than the latter there were
two artifacts: the master could not read events from binlog;
show master events did not show.

Fixed with 
- fragmenting the used io-cached buffer into pieces each size of less
  than max_allowed_packet (bug#30435)
- incrementing show-binlog-events handling thread's max_allowed_packet
  with the max estimated for the replication header size