* postgres_healthcheck: validate once per minute instead of 30 sec
* postgres_driver.nim: change MaxNumCons from 5 to 50
* postgres_driver.nim: split connPool into writeConPool and readConPool
This aims to avoid clashes in insert and select queries
because the inserts and selects can happen concurrently
in relay and store events, respectively.
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 FB