Part of #7497, closes https://github.com/neondatabase/neon/issues/8890.
## Problem
Since leases are in-memory objects, we need to take special care of them
after pageserver restarts and while doing a live migration. The approach
we took for pageserver restart is to wait for at least lease duration
before doing first GC. We want to do the same for live migration. Since
we do not do any GC when a tenant is in `AttachedStale` or
`AttachedMulti` mode, only the transition from `AttachedMulti` to
`AttachedSingle` requires this treatment.
## Summary of changes
- Added `lsn_lease_deadline` field in `GcBlock::reasons`: the tenant is
temporarily blocked from GC until we reach the deadline. This
information does not persist to S3.
- In `GCBlock::start`, skip the GC iteration if we are blocked by the
lsn lease deadline.
- In `TenantManager::upsert_location`, set the lsn_lease_deadline to
`Instant::now() + lsn_lease_length` so the granted leases have a chance
to be renewed before we run GC for the first time after transitioned
from AttachedMulti to AttachedSingle.
Signed-off-by: Yuchen Liang <yuchen@neon.tech>
Co-authored-by: Joonas Koivunen <joonas@neon.tech>
1708743e78