Several more database-y refactors

- DbConnectionFactory renamed to "Database", will now be the primary entry point for DB stuff
- Created IPKConnection interface mostly containing async extensions to IDbConnection, use this going forward
- Reworked the Connection/Command wrappers (that have performance/logging extensions)
- Probably more stuff that I forgot???

PluralKit.Core/Database/Wrappers/IPKCommand.cs

@@ -0,0 +1,17 @@
+using System;
+using System.Data;
+using System.Data.Common;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace PluralKit.Core
+{
+    public interface IPKCommand: IDbCommand, IAsyncDisposable
+    {
+        public Task PrepareAsync(CancellationToken ct = default);
+        public Task<int> ExecuteNonQueryAsync(CancellationToken ct = default);
+        public Task<object> ExecuteScalarAsync(CancellationToken ct = default);
+        public Task<DbDataReader> ExecuteReaderAsync(CancellationToken ct = default);
+        public Task<DbDataReader> ExecuteReaderAsync(CommandBehavior behavior, CancellationToken ct = default);
+    }
+}