Handle multiple SETTINGS_HEADER_TABLE_SIZE in incoming SETTINGS frame

Previously we just assumed that if same settings ID is found in
SETTINGS, it is enough to process last seen entry.  But it turns out
it is not enough for SETTINGS_HEADER_TABLE_SIZE.  If we have 0 and
4096 for SETTINGS_HEADER_TABLE_SIZE in one SETTINGS, we must first
shrink dynamic table to 0 and then enlarge it to 4096.  This means
that we have to remember the minimum value and last value.