When this error happens in the context of a backup it can be a bit mystifying as to why the backup is failing. Add some hints to get the user started.
These hints will appear any time a WAL segment can't be found, which makes the hint about the check command redundant when the user is actually running the check command, but it doesn't seem worth trying to exclude the hint in that case.
Suggested by Hans-Jürgen Schönig.
This new implementation should behave exactly like the old Perl code with the exception of a few updated log messages.
Remove as much of the Perl code as possible without breaking other commands.
The C local is only used for C commands in the main process.
Some tweaking of the existing protocolGet() command was required. Originally the idea was to share the function for local and remote requests but the differences (as in Perl) were too great to make that practical.
