Implement Extraction of WAL Segment General Portion Headers

[sh-soltanpour]

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

As the beginning part of #296, we need to parse the general portion of WAL segment headers. This issue is just for parsing the general header and the rest of the data will be handled in the two other issues.

Describe the solution you'd like
Based on the image below, each page has a fixed-length general header, so we want to parse them and create a list of XLogRecord structs.

source

Additional information

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