* config,restore: introduced `[mydumper] batch-size`

Removed `[tikv-importer] batch-size` to avoid confusion.

Removed `[mydumper] min-region-size` since it is useless now.

* restore,mydump: pre-allocate engine IDs

* restore: separate table checkpoints and engine checkpoints

* importer: stop exposing the UUID

* checkpoints: make checkpoint diff understand 1 table = many engines

* checkpoints: make file checkpoints recognize multiple engines

* checkpoints: migrated MySQL-based checkpoint to multi-engine as well

* restore: adapt restore workflow for multi-engine

* tests: added test case for multi-engine

* *: fixed code

* *: addressed comments

* *: addressed comments

* Support non-uniform batch size (pingcap#114)

* mydump: non-uniform batch size

* *: make the `batch-size-scale` configurable

* *: implemented the optimized non-uniform strategy

* tests: due to change of strategy, checkpoint_engines count becomes 4 again

* mydump/region: slightly adjust the batch size computation

* Use the exact result of 1/Beta(N, R) instead of an approximation
* When the number of engines is small and the total engine size of the
  first (table-concurrency) batches exceed the table size, the last batch
  was truncated, and disrupt the pipeline. Now in these case we will
  reduce the batch size to avoid this disruption.

* restore: log the SQL size and KV size of each engine for debugging

* config: change default batch size and ratio given experiment result

* config: added more explanation about batch-import-ratio

Co-authored-by: Lonng <chris@lonng.org>