Stamp 2.4

Fix an oversight in format 2: nextlsn

Format 2 (1b0cbac) forgot to add
nextlsn. Format 1 includes this information if include-lsn is provided.
This information can be useful for applications that use LSN position as
a restart point.

Add macro for backward compatibility

UInt64GetDatum was introduced in 9.6. I forgot to test old stable
versions and didn't get it. Fortunately, @df7cb reported it.

Use correct format specifier

uint64 should use UINT64_FORMAT instead of %lu because format specifier
depends on a type probe. It suppresses a compiler warning.

Adjust instructions

Use default max_replication_slots value as suggestion for examples /
tests.

Improve style