Provide interfaces to store and fetch undo records.

Provide interfaces to store and fetch undo records.

Add the capability to form undo records and store them in undo logs.  We
also provide the capability to fetch the undo records.  This layer will use
undo-log-storage to reserve the space for the undo records and buffer
management routines to write and read the undo records.

Undo records are stored in sequential order in the undo log.  Each undo
record consists of a variable length header, tuple data, and payload
information.  The undo records are stored without any sort of alignment
padding and a undo record can span across multiple pages.  The undo records
for a transaction can span across multiple undo logs.

Author: Dilip Kumar with contributions from Robert Haas, Amit Kapila,
        Thomas Munro, Vignesh C and Rafia Sabih
Reviewed-by: Earlier version of this patch is reviewed by Amit Kapila
Tested-by: Neha Sharma
Discussion: https://www.postgresql.org/message-id/CAFiTN-uVxxopn0UZ64%3DF-sydbETBbGjWapnBikNo1%3DXv78UeFw%40mail.gmail.com