Dynamic endpoints handlers

chttpd hardcoded handlers are replaced with with dynamic url handlers,
which are functions in a dynamically created module, assembled from
priv/chttpd_handler.cfg files from all interested applications.

For couched these are currently: chttpd, mem3 and global_changes.

This is a special branch that pull the prepared branches (of the same name)
of chttpd, mem3 and global_changes, which have the config files.

Run 'make check' to test the handlers. They are implicitly tested by other
tests and there is a whitebox callback test chttpd_handler_callback_test,
which calls the dynamic functions directly and mocks the funs that they
return. This tests precisely the relationship between the *_handler function
clauses and the returned function.

There are more tests to come, both testing the endpoints via http, and the
configuration assembly.

BugzID: 27037
The value in splitting this out is that we will know the full pass/fail matrix for each of the tests. Keeping them together like this means we'll only know the first failure. I think that's a useful change if it doesn't make the test too much more complicated. I don't think it's *required* to split these out given how unlikely breaking one (but not all) is, so I'm +1 on this test as it stands and +1 on improving it if possible.