Reformat the postgres connection URL to work properly with unix socke…

…t connections (Zondax#150)

I ran into a problem trying to connect `namadexer` to a postgres
database using a Unix domain socket rather than a network hostname. This
is necessary in order to run the `server` in environments like Google
Cloud Run, which exposes connections to Postgresql as a local domain
socket.

**The problem:**
* Google Cloud SQL creates a socket with a path like
`/cloudsql/namada:us-central1:namadexer-db-instance`
* Setting
`DATABASE_HOST=/cloudsql/namada:us-central1:namadexer-db-instance` would
produce a connection URL like the following:
    ```

postgres://postgres:wow@/cloudsql/namada:my-region:namadexer-db-instance:5432/blockchain
    ```
* This URL is not valid.

**The solution:**
* There is an alternate form for the postgres URL, like this:
    ```

postgres://postgres:wow@dummy-hostname:5432/blockchain?host=/cloudsql/namada:my-region:namadexer-db-instance
    ```
* In this case, the actual `host` can contain `/` and `:` characters
just fine.
* The `dummy-hostname` in the host part of the URL must be present for
the URL to be parsed properly, but is ignored.

src/database.rs

@@ -69,17 +69,25 @@ impl Database {
         // sqlx expects config of the form:
         // postgres://user:password@host:port/db_name
         let config = format!(
-            "postgres://{}:{}@{}/{}",
-            db_config.user,
-            db_config.password,
-            db_config.host,
-            db_config.dbname
+            // In order to accommodate the use of a unix socket, we are actually supplying a dummy
+            // string "not-the-host" to the host portion of the URL, and putting the actual host in the
+            // query string. This is the only way to provide a "host" to sqlx that contains slashes.
+            // When "host=" is given as a query parameter, postgres ignores the host portion of the URL
+            // though it is still required to be present for the URL to parse correctly.
+            "postgres://{}:{}@not-the-host:{}/{}?host={}",
+            db_config.user, db_config.password, db_config.port, db_config.dbname, db_config.host
         );
 
         // If timeout setting is not present in the provided configuration,
         // lets use our default timeout.
         let timeout = db_config.connection_timeout.unwrap_or(DATABASE_TIMEOUT);
 
+        debug!(
+            "connecting to database at {} with timeout {}",
+            config.replace(&db_config.password, "*****"),
+            timeout
+        );
+
         let pool = PgPoolOptions::new()
             .max_connections(10)
             .acquire_timeout(Duration::from_secs(timeout))