## Problem
Prefetch is performed locally, so different backers can request the same
pages form PS.
Such duplicated request increase load of page server and network
traffic.
Making prefetch global seems to be very difficult and undesirable,
because different queries can access chunks on different speed. Storing
prefetch chunks in LFC will not completely eliminate duplicates, but can
minimise such requests.
The problem with storing prefetch result in LFC is that in this case
page is not protected by share buffer lock.
So we will have to perform extra synchronisation at LFC side.
See:
https://neondb.slack.com/archives/C0875PUD0LC/p1736772890602029?thread_ts=1736762541.116949&cid=C0875PUD0LC
@MMeent implementation of prewarm:
See https://github.com/neondatabase/neon/pull/10312/
## Summary of changes
Use conditional variables to sycnhronize access to LFC entry.
---------
Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>
b1d8771d5f