This commit is a correction to nodes.h to maintain values assigned to
node tags as same as those for PostgreSQL.   It is effective to allow
PostgreSQL extension binaries to run within Postgres-XC.

Proposed by Krzysztof Nienartowicz.
Improves pgxc_ctl behavior to

1) When issuing pg_ctl start.   Added -w option for better synchronization.

2) At pinging coordinator/datanode, when ping does not detect the node,
   retries three times with 100 millisecond interval just in case.
Fixes incorrest nodePing() return value evaluation when
initializing a coordinator slave.