Before this patch struct tuple had two boolean bit fields: is_dirty and
has_uploaded_refs. It is worth mentioning that sizeof(boolean) is
implementation depended. However, in code it is assumed to be 1 byte
(there's static assertion restricting the whole struct tuple size by 10
bytes). So strictly speaking it may lead to the compilation error on
some non-conventional system. Secondly, bit fields anyway consume at
least one size of type (i.e. there's no space benefits in using two
uint8_t bit fields - they anyway occupy 1 byte in total). There are
several known pitfalls concerning bit fields:
 - Bit field's memory layout is implementation dependent;
 - sizeof() can't be applied to such members;
 - Complier may raise unexpected side effects
   (https://lwn.net/Articles/478657/).

Finally, in our code base as a rule we use explicit masks:
txn flags, vy stmt flags, sql flags, fiber flags.

So, let's replace bit fields in struct tuple with single member called
`flags` and several enum values corresponding to masks (to be more
precise - bit positions in tuple flags).

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