Add LSN <-> time conversion functionality

Hi,

Elsewhere [1] I required a way to estimate the time corresponding to a
particular LSN in the past. I devised the attached LSNTimeline, a data
structure mapping LSNs <-> timestamps with decreasing precision for
older time, LSN pairs. This can be used to locate and translate a
particular time to LSN or vice versa using linear interpolation.

I've added an instance of the LSNTimeline to PgStat_WalStats and insert
new values to it in background writer's main loop. This patch set also
introduces some new pageinspect functions exposing LSN <-> time
translations.

Outside of being useful to users wondering about the last modification
time of a particular block in a relation, the LSNTimeline can be put to
use in other Postgres sub-systems to govern behavior based on resource
consumption -- using the LSN consumption rate as a proxy.

As mentioned in [1], the LSNTimeline is a prerequisite for my
implementation of a new freeze heuristic which seeks to freeze only
pages which will remain unmodified for a certain amount of wall clock
time. But one can imagine other uses for such translation capabilities.

The pageinspect additions need a bit more work. I didn't bump the
pageinspect version (didn't add the new functions to a new pageinspect
version file). I also didn't exercise the new pageinspect functions in a
test. I was unsure how to write a test which would be guaranteed not to
flake. Because the background writer updates the timeline, it seemed a
remote possibility that the time or LSN returned by the functions would
be 0 and as such, I'm not sure even a test that SELECT time/lsn > 0
would always pass.

I also noticed the pageinspect functions don't have XML id attributes
for link discoverability. I planned to add that in a separate commit.

- Melanie

[1] https://www.postgresql.org/message-id/CAAKRu_b3tpbdRPUPh1Q5h35gXhY%3DspH2ssNsEsJ9sDfw6%3DPEAg%40mail.gmail.com