tuple: enable isolated JSON updates

Isolated tuple update is an update by JSON path, which hasn't a
common prefix with any other JSON update operation in the same
set. For example, these JSON update operations are isolated:

    {'=', '[1][2][3]', 100},
    {'+', '[2].b.c', 200}

Their JSON paths has no a common prefix. But these operations are
not isolated:

    {'=', '[1][2][3]', 100},
    {'+', '[1].b.c', 200}

They have a common prefix '[1]'.

Isolated updates are a first part of fully functional JSON
updates. Their feature is that their implementation is relatively
simple and lightweight - an isolated JSON update does not store
each part of the JSON path as a separate object. Isolated update
stores just string with JSON and pointer to the MessagePack
object to update.

Such isolated updates are called 'bar update'. They are a basic
brick of more complex JSON updates.

Part of #1261