Implement the ability to extract WAL records from WAL files

[sh-soltanpour]

Is your feature request related to a problem? Please describe.
This is the first part of the Wal infrastructure project (#288). This is a part of a bigger feature (parsing a WAL file into an understandable list of records).

In this issue, we want to be able to parse the WAL file into WAL records, then (probably in another issue), will parse each WAL record into header and body, and then based on each record's header (like XLogRecord struct in the proposal), we can parse the body into a C struct and extract data from it.

Describe the solution you'd like
We need to parse both complete and partial WAL files. I need to check what are the differences between the structures of these two types of files. Then, I will find each record's header inside the file (probably using pre-defined offset sizes), then we can interpret the rest of the record based on them.