Add redo LSN to pgstats files

Add more debugging information when failing to read pgstats files

Make write of pgstats file durable at shutdown

Add assertion in pgstat_write_statsfile() about processes allowed

injection_points: Add some fixed-numbered statistics

injection_points: Add some cumulative stats for injection points

Add helper routines to retrieve data for custom fixed-numbered pgstats

Introduce pluggable APIs for Cumulative Statistics

Switch PgStat_Kind from an enum to a uint32 type

Remove PgStat_KindInfo.named_on_disk