phpx/server: improve wire format

This commit updates the wire format of the phpx server. Instead of being
string-based, messages sent back and forth between go and php are now
base64-encoded DEFLATEd strings. This makes them a lot smaller and
faster to send.

internal/phpx/errors.go

@@ -6,6 +6,7 @@ import "fmt"
 const (
 	errInit    = "Server initialization failed"
 	errClosed  = "Server closed"
+	errSend    = "Failed to encode request"
 	errReceive = "Failed to decode response"
 )