Improve handling of concurrent node modifications

consider the following scenario:

• session1 modifies node /a by adding a child node b
• session2 modifies node /a by adding a child node c
• session2 saves its changes
• session1 tries to save its changes which results in a InvalidItemStateException

this behavior is in accordance with the spec. the spec however doesn't prevent a
more lenient handling of this scenario.

if the concurrent modifications are non-conflicting and trivial the changes could
be silently merged in session1's transient state of node /a.

examples for trivial non-conflicting changes:

• s1 adds child node a, s2 removes child node b
• s1 adds child node a, s2 adds child node b
• s1 adds child node a, s2 adds mixin type

examples for non-trivial and/or conflicting changes:

• s1 removes child node a, s2 removes child node a
• s1 adds child node a, s2 adds child node a
• s1 adds sns child node a (> a[3]), s2 adds sns child node a (> a[3])
• s1 adds sns child node a (-> a[3]), s2 removes sns child node a[1]
• s1 removes sns child node a[2], s2 reorders child nodes affecting sns nodes a