+ Starting from this commit, all return types for tasks will use style similar to most documentations featured on docs.microsoft.com

References:
https://docs.microsoft.com/en-us/dotnet/api/microsoft.entityframeworkcore.dbcontext.-ctor?view=efcore-2.1
https://docs.microsoft.com/en-us/dotnet/api/system.io.filestream.readasync?view=netcore-2.1
https://docs.microsoft.com/en-us/dotnet/api/system.io.textwriter.writelineasync?view=netcore-2.1#System_IO_TextWriter_WriteLineAsync_System_Char___
And many more other asynchronous method documentations featured in the latest BCL.

* ConnectAsync(bool external) API implemented

This allows developers to handle the AudioClient externally (for example with Lavalink)

* Modify ConnectAsync API and add DisconnectAsync to IAudioChannel

* Update summary message on IAudioChannel.DisconnectAsync()

* Review changes:
- Fix `if (!external)` styling
- Remove unecessary ConnectAsync overload

* SocketVoiceChannel overload update

* Update IAudioChannel.ConnectAsync()
- Remove ConfigAction from parameters
- Add SelfDeafen/SelfMute to parameters

* Remove SocketVoiceChannel.ConnectAsync() default params (Inherit)

* Fix #995 ICategoryChannel.CategoryID throws NotSupportedException

* Add tests

* change run mode of TestChannelCategories

* Add throw for GetCategoryAsync

* Add xml doc explaining why exception is thrown

* Add test coverage for text and voice channel categories

* initial implementation of INestedChannel

* more implementation of INestedChannel design

* Add case in RestChannel Create for Category type

* set the CategoryID for RestVoiceChannel

* rewrite channel category tests to work with existing pattern

* remove outdated todo

* Make IVoiceChannel implement INestedChannel

* remove redundant interface implementation

* Add c#7 feature from feedback

* Remove redundant GetCategoryAsync methods from socket entities

* Added configureawait to async methods

* change signature of interface GetCategoryAsync

* Add check for cachemode in rest channel GetCategory

* remove redundant IGuildChannel interface from ITextChannel and IVoiceChannel