mode="asynchronous"should be the default, in Orbeon Forms
mode="synchronous"is the default.
xforms-model-construct-done, two background submissions are started. The
<xxf:join-submissions>action waits until they both have completed.
xxf:cache="true"and already in cache, it is immediately retrieved and no background submission takes place._
replace="all" are not supported in asynchronous mode.
file:are supported) runs in a separate execution thread