fix(v2): prevent stale channel messages in MultiTree concurrency

[songgaoye]

SaveVersionConcurrently, SnapshotConcurrently, and WarmLeaves previously
used shared errorCh/doneCh channels. Surplus messages could remain in the
buffers and be consumed by later calls, causing version mismatches or
incorrect results.

Use per-call local result channels (one message per goroutine) to avoid
cross-call contamination. Remove the unused doneCh/errorCh fields and
saveVersionResult type, and have WarmLeaves collect all errors.

Add tests covering: single-tree error, all-trees error,
sequential
calls after recovery, SnapshotConcurrently error,
WarmLeaves error
collection, and happy path. Use tableSchema() to
reconstruct dropped
tables from sqlite_master rather than hardcoding column
definitions.