[PATCH] Generic type subscription

Hi,

Regarding to the previous conversations [1], [2], here is a patch (with some
improvements from Nikita Glukhov) for the generic type subscription. It
allows
to define type-specific subscription logic for any data type and has
implementations for the array and jsonb types. There are following changes
in this
patch:

* A new column for `pg_type` (`regproc typsubscription`) which points out a
  type-specific subscription procedure for particular data type. It can be
none
  (and it's a default value), which means that this data type doesn't
support
  subscription.

* Type-specific code (e.g. any kind of verification, type coercion, actual
  data extraction and update) in the array subscription implementation was
  separated from generic code into `array_subscription` procedure. Generic
  implementation assumes that subscription expression can be in form `[a]`
or
  `[a:b]`, and there isn't any restrictions for type of `a` and `b`.

* Using the same api a new subscription logic was implemented for `jsonb`
type
  in `jsonb_subscription` procedure. Several changes were introduced into
jsonb
  functions just to be able to share common code.

I believe that this patch is more or less in good shape, so I would like to
know
what do you think about it? Feedback is welcome.

[1]:
https://www.postgresql.org/message-id/flat/CA+q6zcV8qvGcDXurwwgUbwACV86Th7G80pnubg42e-p9gsSf=g@mail.gmail.com#CA+q6zcV8qvGcDXurwwgUbwACV86Th7G80pnubg42e-p9gsSf=g@mail.gmail.com
[2]:
https://www.postgresql.org/message-id/flat/CA+q6zcX3mdxGCgdThzuySwH-ApyHHM-G4oB1R0fn0j2hZqqkLQ@mail.gmail.com#CA+q6zcX3mdxGCgdThzuySwH-ApyHHM-G4oB1R0fn0j2hZqqkLQ@mail.gmail.com