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/Database.cs

@@ -0,0 +1,52 @@
+using System;
+using System.Threading.Tasks;
+
+using App.Metrics;
+
+using Npgsql;
+
+using Serilog;
+
+namespace PluralKit.Core
+{
+    public class Database
+    {
+        private readonly CoreConfig _config;
+        private readonly ILogger _logger;
+        private readonly IMetrics _metrics;
+        private readonly DbConnectionCountHolder _countHolder;
+
+        public Database(CoreConfig config, DbConnectionCountHolder countHolder, ILogger logger,
+                        IMetrics metrics)
+        {
+            _config = config;
+            _countHolder = countHolder;
+            _metrics = metrics;
+            _logger = logger;
+        }
+
+        public async Task<IPKConnection> Obtain()
+        {
+            // Mark the request (for a handle, I guess) in the metrics
+            _metrics.Measure.Meter.Mark(CoreMetrics.DatabaseRequests);
+            
+            // Create a connection and open it
+            // We wrap it in PKConnection for tracing purposes
+            var conn = new PKConnection(new NpgsqlConnection(_config.Database), _countHolder, _logger, _metrics);
+            await conn.OpenAsync();
+            return conn;
+        }
+        
+        public async Task Execute(Func<IPKConnection, Task> func)
+        {
+            await using var conn = await Obtain();
+            await func(conn);
+        }
+        
+        public async Task<T> Execute<T>(Func<IPKConnection, Task<T>> func)
+        {
+            await using var conn = await Obtain();
+            return await func(conn);
+        }
+    }
+}