* archive: move error to trace level when insert row fails
That is helpful to prevent the node to spam the logs when it shares
connection to the same Postgres database with other nodes, in
which case the following log appears too much:
topics="waku archive" tid=1 file=archive.nim:113 err="error in
runStmt: error in dbConnQueryPrepared calling waitQueryToFinish: error
in query: ERROR: duplicate key value violates unique constraint
"messageindex" DETAIL: Key
(messagehash)=(88f4ee115eef6f233a7dceaf975f03946e18666adda877e38d61be98add934e8)
already exists. "
* 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.
