Fix the connection release bug when occurred an exception.

src/DotNetCore.CAP.MySql/MySqlStorageConnection.cs

@@ -171,6 +171,7 @@ SELECT * FROM `{_prefix}.received` WHERE Id=LAST_INSERT_ID();";
             catch (MySqlException)
             {
                 transaction.Dispose();
+                connection.Dispose();
                 throw;
             }
 

src/DotNetCore.CAP.PostgreSql/PostgreSqlStorageConnection.cs

@@ -146,6 +146,7 @@ namespace DotNetCore.CAP.PostgreSql
             catch (NpgsqlException)
             {
                 transaction.Dispose();
+                connection.Dispose();
                 throw;
             }
 

src/DotNetCore.CAP.SqlServer/SqlServerStorageConnection.cs

@@ -153,6 +153,7 @@ VALUES(@Name,@Group,@Content,@Retries,@Added,@ExpiresAt,@StatusName);";
             catch (SqlException)
             {
                 transaction.Dispose();
+                connection.Dispose();
                 throw;
             }