Add "documentation" flags for WriteTransaction

Add "documentation" flags for WriteTransaction

In WriteTransaction, the createMissingParents argument to put() and
merge() is a boolean, which results in code like

    ...put(store, path, data, true);

which isn't immediately readable to neophytes. This patch adds two
constants, CREATE_MISSING_PARENTS and FAIL_ON_MISSING_PARENTS, so that
client code can be written as

    ...put(store, path, data, CREATE_MISSING_PARENTS);

instead (but without introducing an enum).

The FAIL_ON_MISSING_PARENTS constant also documents the behaviour in
the other path; I named it based on the pre-condition check in
AbstractWriteTransaction (which results in an IllegalArgumentException
if a parent is missing).

Change-Id: Ic95b5e4f3d8574f0014cdf6a00a77013b94b00cc
Signed-off-by: Stephen Kitt <skitt@redhat.com>