[Rust] [DataFusion] MemTable::load() should load partitions in parallel

[asfimport]

MemTable::load() should load partitions in parallel using async tasks, rather than loading one partition at a time.

Also, we should make batch size configurable. It is currently hard-coded to 1024*1024 which can be quite inefficient.

Reporter: Andy Grove / @andygrove
Assignee: Andy Grove / @andygrove

PRs and other links:

• GitHub Pull Request #8428

_{Note: This issue was originally created as ARROW-10251. Please see the migration documentation for further details.}

[asfimport]

Andy Grove / @andygrove:
Issue resolved by pull request 8428
#8428