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 <[email protected]>
Author: Oleg Bartunov <[email protected]>
Author: Nikita Glukhov <[email protected]>
Author: Nikita Malakhov <[email protected]>

Discussion: https://www.postgresql.org/message-id/flat/[email protected]

Author: Nikita Malakhov