Versioned rows in TOASTed values for Default Toaster support

Original TOAST mechanics does not support rows versioning
for TOASTed values.
Toaster snapshot - refactored generic toaster, implements
rows versions check in toasted values to share common parts
of toasted values between different versions of rows.

Author: Teodor Sigaev <teodor@sigaev.ru>
Author: Oleg Bartunov <obartunov@postgrespro.ru>
Author: Nikita Glukhov <n.gluhov@postgrespro.ru>
Author: Nikita Malakhov <n.malakhov@postgrespro.ru>

Discussion: https://www.postgresql.org/message-id/flat/224711f9-83b7-a307-b17f-4457ab73aa0a@sigaev.ru

Author: Nikita Malakhov