Add context.Context to Logger interface

This allows custom logger adapters to add additional fields to log
messages. For example, a HTTP server may with to log the request ID.

fixes #428

bench_test.go

@@ -201,7 +201,8 @@ func BenchmarkSelectWithoutLogging(b *testing.B) {
 
 type discardLogger struct{}
 
-func (dl discardLogger) Log(level pgx.LogLevel, msg string, data map[string]interface{}) {}
+func (dl discardLogger) Log(ctx context.Context, level pgx.LogLevel, msg string, data map[string]interface{}) {
+}
 
 func BenchmarkSelectWithLoggingTraceDiscard(b *testing.B) {
 	conn := mustConnect(b, mustParseConfig(b, os.Getenv("PGX_TEST_DATABASE")))