box: introduce offset_slot cache in key_part

tuple_field_by_part looks up the tuple_field corresponding to the
given key part in tuple_format in order to quickly retrieve the offset
of indexed data from the tuple field map. For regular indexes this
operation is blazing fast, however of JSON indexes it is not as we
have to parse the path to data and then do multiple lookups in a JSON
tree. Since tuple_field_by_part is used by comparators, we should
strive to make this routine as fast as possible for all kinds of
indexes.

This patch introduces an optimization that is supposed to make
tuple_field_by_part for JSON indexes as fast as it is for regular
indexes in most cases. We do that by caching the offset slot right in
key_part. There's a catch here however - we create a new format
whenever an index is dropped or created and we don't reindex old
tuples. As a result, there may be several generations of tuples in the
same space, all using different formats while there's the only key_def
used for comparison.

To overcome this problem, we introduce the notion of tuple_format
epoch. This is a counter incremented each time a new format is
created. We store it in tuple_format and key_def, and we only use
the offset slot cached in a key_def if it's epoch coincides with the
epoch of the tuple format. If they don't, we look up a tuple_field as
before, and then update the cached value provided the epoch of the
tuple format.

Part of #1012