core (xmpp): make `send` a blocking method, fix `sendMessageData` calls: original `send` method is blocking, and it is used as such by Wokkel and thus can't be changed to an async method easily. However, an Async method is necessary to have an async trigger at the very end of the send workflow for end-to-end encryption. To workaround that, `send` is an async method which call `a_send`, an async method which actually does the sending. This way legacy method can still call `send` while `a_send` can be await otherwise. Fix calls to `sendMessageData`: the method now being an `async` one, `ensureDeferred` had to be used in some calls.

# Minimal makefile for Sphinx documentation
#

# You can set these variables from the command line.
SPHINXOPTS    =
SPHINXBUILD   = sphinx-build
SOURCEDIR     = .
BUILDDIR      = .build

# Put it first so that "make" without argument is like "make help".
help:
	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)

.PHONY: help Makefile

# Catch-all target: route all unknown targets to Sphinx using the new
# "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
%: Makefile
	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)