JSS and JSSP script pooling

[obiltschnig]

In the current implementation JavaScript servlets and server pages are compiled whenever they are requested. From a performance perspective this is unacceptable. There should be a pooling mechanism that enables reuse of compiled scripts.

[obiltschnig]

Servlet is now compiled only on first load. However, there now can only be a single request to a servlet at a given time (processing of servlets is serialized, protected by mutex). This is fine for embedded web servers that don't have to serve many concurrent users, so we'll postpone this issue.