make local logs valid

[carlsverre]

Currently, graft local logs are fragmented due to concurrent writers. But a recent bug discovered in the snapshot code means my solution for concurrent writes was fundamentally wrong anyway.

So I need a new solution for local logs + concurrent write/push/pull ops.

Raw brain dump on the current state. Probably will only make sense to me.

volume foo
read:
 local { (last sync point).next() ..= latest local lsn }
 remote { LSN::FIRST ..= latest remote lsn }

after push:
read:
 remote { LSN::FIRST ..= latest remote lsn }

write
 local { (last sync point).next() ..= latest local lsn }
 remote { LSN::FIRST ..= latest remote lsn }

local log on client A:
 1..=5 -> pushed to remote 1
 .. pulled lsn 5 from remote due to concurrent writes
equivalent to git rebase remote:5, but dropping any commits that we
integrated: 1..=5, now local is just a "pointer" to a empty state
on top of remote:5
 6..=8 -> based on remote lsn 5 pushed to remote 6

if you try to read only from the local log:
 1..=8
-> this is corrupt

 local 6..=8
 remote 2..=5
 local 1..=5 == remote 1

 local 6..=8
 remote 1..=5

-- new branch model

read: lookup local branch
 [local { LSN::FIRST ..= latest local lsn }]
 remote { LSN::FIRST ..= latest remote lsn }

write: lookup local branch
 -> create new local branch
 -> write to the local branch

push:
 -> get local branch
write:
 -> get local branch
push:
 -> push it to the remote
write:
 -> commit to local branch
pull:
 -> push it to the remote


after push:
read:
 remote { LSN::FIRST ..= latest remote lsn }

write
 local { (last sync point).next() ..= latest local lsn }
 remote { LSN::FIRST ..= latest remote lsn }