Epic: Support pinned readonly nodes

[LizardWizzard]

The feature allows to start a compute node pinned at particular LSN. Currently it is possible to start such a node with zenith pg start <name_of_the_branch>@<lsn>.

Missing pieces:

• Pageserver is not aware of pinned readonly nodes and can garbage collect data that might be in scope of running instance #883
• Lsn at which readonly node is started is not validated, it can be in the future (pageserver will wait for future lsn which might result in a timeout), it can be pre initdb lsn. #884
• Handling of readonly nodes in zenith cli is incomplete, when node is started it cannot be restarted etc. #885
• Read only compute pinned to an lsn requests page at a future lsn #3532
• Another failure when doing pg_dump off of pinned compute #3596

Feel free to add to this list

[LizardWizzard]

@vadim2404 should we merge this with read-only replica efforts tracking?

[stepashka]

@shanyp , isn't this already supported?
what is the status of teh tasks listed here, are they complete, not relevant anymore or smth else?

[stepashka]

is this about the storage, compute or both? @vadim2404 @shanyp

[vadim2404]

it's about compute. It seems that we had this capability from the very beginning, but no one tested it.

[LizardWizzard]

Tests exist in neon repo. And feature is available in neon_local