v19.9.0rc1

This release contains some stability improvements and enables serializing BucketListDB indexes on disk.

Stability improvements

• Tests: allow overlay to work automatically in loopback mode

New features

• Serialize indexes to disk
  This change saves BucketListDB indexes on disk, so validators can load the index files, instead of reindexing the entire BucketList on startup. On EBS, indexing the BucketList takes about 4 minutes, but loading indexes takes about 1-2 seconds only.

Soroban (only enabled in vNext, not enabled in default build)

• Fix reporting of base XDR git version (was reporting "next")
• Properly setup rustup in dev containers
• bump env
• bump env
• Add support for diagnostic events
• Preliminary value overhaul change
• Adapt SnapshotSource interface
• Fixup xdr hash script
• Allow multiple host fn invocations in the same transaction.