Implement extraction of the header part of the data portion in WAL segments

[sh-soltanpour]

Is your feature request related to a problem? Please describe.

As a part of #296, we need to parse the header part of data portion from the image below. To do this, we probably need to get some information from what we already have from XLogRecord (parsed in #299).

source

Describe the solution you'd like

The header part contains zero or more XLogRecordBlockHeaders and zero or one XLogRecordDataHeaderShort (or XLogRecordDataHeaderLong). It must contain at least one of these.

The solution needs to understand the type structs used in the header part, and parse the page into the appropriate structs. Some records may be stored in multiple pages so the solution must be able to skip the pages with continuous data to reach the next header.

Additional information

Any additional information you can provide, like an overall design description