[FIXED] Default to allowing binary stream snapshots

[MauriceVanVeen]

When routes to other servers are established binarySnapshots is initially set to false indicating it is not supported. Then once a STATSZ is received binarySnapshots will be updated to true.

Whether this server supports binary snapshots is determined by looping over the peers and checking if other servers don't support it:

if sir, ok := s.nodeToInfo.Load(p.ID); ok && sir != nil && !sir.(nodeInfo).binarySnapshots

However, there's an issue here as the server will shortly think binarySnapshots isn't supported and go back to the legacy snapshot which contains interior deletes as a []uint64 which will be disastrous if a stream has a huge amount of interior deletes. Since binary stream snapshots are supported since 2.10.0, we can simply flip the binarySnapshots default from false to true.

Alternative to #7476

Signed-off-by: Maurice van Veen github@mauricevanveen.com

[neilalexander]

Principally OK with me, with the caveat that this will make one-shot downgrades to 2.9.x or earlier potentially unsafe. Will await @derekcollison's opinion too.

[wallyqs]

LGTM

[wallyqs]

include warning from @alexbozhenko branch? a4a17f4

[derekcollison]

LGTM

For downgrade to 2.9, what would actually happen? Can we rebuild and ask for a new one in old style if we do not know what it is?

[MauriceVanVeen]

For downgrade to 2.9, what would actually happen? Can we rebuild and ask for a new one in old style if we do not know what it is?

Binary snapshots have been introduced since 2.10.0, so this PR doesn't change what would happen if binary snapshots exist, but:
For example, when going from 2.10.0 that contains binary snapshots to 2.9.x, I think the 2.9 server would already not understand the binary snapshot and break?