Use Diesels MultiConnections Derive

With this PR we remove allmost all custom macro's to create the multiple
database type code. This is now handled by Diesel it self.

This removed the need of the following functions/macro's:
- `db_object!`
- `::to_db`
- `.from_db()`

All `conn` variables do not need a `mut` anymore (says nightly rust)

It is also possible to just use one schema instead of multiple per type.

Cargo.toml

@@ -13,7 +13,11 @@ publish = false
 build = "build.rs"
 
 [features]
-# default = ["sqlite"]
+default = [
+    # "sqlite",
+    # "postgresql",
+    # "mysql",
+]
 # Empty to keep compatibility, prefer to set USE_SYSLOG=true
 enable_syslog = []
 mysql = ["diesel/mysql", "diesel_migrations/mysql"]
@@ -28,7 +32,7 @@ enable_mimalloc = ["mimalloc"]
 # It enables the usage of the diesel_logger crate, which is able to output the generated queries.
 # You also need to set an env variable `QUERY_LOGGER=1` to fully activate this so you do not have to re-compile
 # if you want to turn off the logging for a specific run.
-query_logger = ["diesel_logger"]
+# query_logger = ["diesel_logger"] # Currently not able to be used with MultiConnection.
 
 # Enable unstable features, requires nightly
 # Currently only used to enable rusts official ip support
@@ -77,7 +81,7 @@ serde_json = "1.0.105"
 # A safe, extensible ORM and Query builder
 diesel = { version = "2.1.1", features = ["chrono", "r2d2"] }
 diesel_migrations = "2.1.0"
-diesel_logger = { version = "0.3.0", optional = true }
+# diesel_logger = { version = "0.3.0", optional = true } # Currently not able to be used with MultiConnection.
 
 # Bundled/Static SQLite
 libsqlite3-sys = { version = "0.26.0", features = ["bundled"], optional = true }