## Problem
We currently offload LFC state unconditionally, which can cause
problems. Imagine a situation:
1. Endpoint started with `autoprewarm: true`.
2. While prewarming is not completed, we upload the new incomplete
state.
3. Compute gets interrupted and restarts.
4. We start again and try to prewarm with the state from 2. instead of
the previous complete state.
During the orchestrated prewarming, it's probably not a big issue, but
it's still better to do not interfere with the prewarm process.
## Summary of changes
Do not offload LFC state if we are currently prewarming or any issue
occurred. While on it, also introduce `Skipped` LFC prewarm status,
which is used when the corresponding LFC state is not present in the
endpoint storage. It's primarily needed to distinguish the first compute
start for particular endpoint, as it's completely valid to do not have
LFC state yet.
- Add ComputeSpec flag `offload_lfc_interval_seconds` controlling
whether LFC should be offloaded to endpoint storage. Default value
(None) means "don't offload".
- Add glue code around it for `neon_local` and integration tests.
- Add `autoprewarm` mode for `test_lfc_prewarm` testing
`offload_lfc_interval_seconds` and `autoprewarm` flags in conjunction.
- Rename `compute_ctl_lfc_prewarm_requests_total` and
`compute_ctl_lfc_offload_requests_total` to
`compute_ctl_lfc_prewarms_total`
and `compute_ctl_lfc_offloads_total` to reflect we count prewarms and
offloads, not `compute_ctl` requests of those.
Don't count request in metrics if there is a prewarm/offload already
ongoing.
https://github.com/neondatabase/cloud/issues/19011
Resolves: https://github.com/neondatabase/cloud/issues/30770
Add `/lfc/(prewarm|offload)` routes to `compute_ctl` which interact with
endpoint storage.
Add `prewarm_lfc_on_startup` spec option which, if enabled, downloads
LFC prewarm data on compute startup.
Resolves: https://github.com/neondatabase/cloud/issues/26343
