Add CoalescePartitionsExec (#272) #1393

Job	Run time
cargo check	8m 36s
Check cargo fmt	49s
build with wasm-pack	2m 38s
cargo test (macos)	10m 18s
cargo test (macos-aarch64)	10m 43s
Verify Vendored Code	1m 3s
cargo test (win64)	35m 32s
cargo test datafusion-cli (amd64)	4m 6s
check Cargo.toml formatting	3m 10s
check configs.md is up-to-date	2m 58s
cargo doc	3m 51s
clippy	5m 19s
cargo test (amd64)	9m 40s
cargo test doc (amd64)	13m 19s
Run sqllogictest with Postgres runner	3m 26s
cargo test pyarrow (amd64)	3m 7s
verify benchmark results (amd64)	11m 20s
	2h 9m 55s

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