Fix segfault when %a is in log_line_prefix and debug message is on.

Fix segfault when %a is in log_line_prefix and debug message is on.

log_line_prefix() gets called to create a log line prefix string. If
"%a" is specified in "log_line_prefix" parameter, log_line_prefix()
calls MASTER_CONNECTION macro, which calls
pool_virtual_master_db_node_id(), which calls ereport(), which calls
log_line_prefix() if debug message is on. This leads to an infinite
recursion and a segfault. Fix is, calling MASTER_NODE_ID macro instead
of MASTER_CONNECTION macro.

Per bug 376.