* removing implicit dependency with libpq if postgres is not being used.
* We only run the postgres tests when explicitly willing to, i.e. make
POSTGRES=1 test. The reason is that the postgres tests expect a
database instance to be running locally.
It starts an asynchronous infinite task that checks the connectivity
with the database. In case of error, the postgres_healthcheck task
tries to reconnect for a while, and if it determines that the connection
cannot be resumed, then it invokes a callback indicating that
situation. For the case of the `wakunode2` app, this callback
quits the application itself and adds a log trace indicating
the connectivity issue with the database.
Ivan Folgueira Bande
Hanno Cornelius