Add support for multiple simultaneous database features by using macros.

Diesel requires the following changes:
- Separate connection and pool types per connection, the generate_connections! macro generates an enum with a variant per db type
- Separate migrations and schemas, these were always imported as one type depending on db feature, now they are all imported under different module names
- Separate model objects per connection, the db_object! macro generates one object for each connection with the diesel macros, a generic object, and methods to convert between the connection-specific and the generic ones
- Separate connection queries, the db_run! macro allows writing only one that gets compiled for all databases or multiple ones

Cargo.toml

@@ -123,6 +123,9 @@ structopt = "0.3.16"
 # Logging panics to logfile instead stderr only
 backtrace = "0.3.50"
 
+# Macro ident concatenation
+paste = "1.0"
+
 [patch.crates-io]
 # Use newest ring
 rocket = { git = 'https://github.com/SergioBenitez/Rocket', rev = '1010f6a2a88fac899dec0cd2f642156908038a53' }