Transaction name should be "<METHOD> unknown route" when no automatic/configured name

From https://github.com/elastic/apm/blob/master/specs/agents/tracing-instrumentation-http.md#http-transactions
> In case a name cannot be automatically determined, and a custom name has not been provided by other means, the transaction should be named unknown route, e.g. POST unknown route. This would normally also apply to requests to unknown endpoints, e.g. the transaction for the request GET /this/path/does/not/exist would be named GET unknown route, whereas the transaction for the request GET /users/123 would still be named GET /users/{id} even if the id 123 did not match any known user and the request resulted in a 404.

I think we should implement https://github.com/elastic/apm-agent-php/issues/136 (Configuration option: USE_PATH_AS_TRANSACTION_NAME) and https://github.com/elastic/apm-agent-php/issues/137 (Configuration option: URL_GROUPS) first to provide users with ability to fallback to the current behavior (transaction name as " "). Implementing transaction naming based on the route will require large effort even only for top 5 most popular PHP web frameworks.