[to #67] support setting api version in br requests

[zz-jason]

Signed-off-by: Jian Zhang zjsariel@gmail.com

What problem does this PR solve?

Issue Number: [to #67]

Problem Description: Not a problem. In order to backup and restore data between clusters with different API versions, we need to indicate tikv-servers about the target encoding format to either API v1 or v2.

BTW, this PR also solves the following problems:

1. the compiled binary is named br which is the same as the backup restore tools for TiDB (https://github.com/pingcap/tidb/tree/master/br). It causes misunderstanding about br.
2. there is a --cf flag for the backup raw command which increases the bar to use brsince users need to understand which column family their data is stored in. This PR removed this flag, making br easier to use.

What is changed and how does it work?

add a flag for users to indicate the destination API version for backup.

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change

Check List for Tests

This PR has been tested by at least one of the following methods:

• Unit test
• Integration test

Side effects

• Increased code complexity, WHY: users need to be aware of the API versions to use BR tools

Related changes

• Need to update the documentation

[zz-jason]

now the help message of tikv-br backup raw is:

➜ ./bin/tikv-br backup raw --help
(experimental) backup a raw kv range from TiKV cluster

Usage:
  br backup raw [flags]

Flags:
      --dst-api-version string   The encoding method of backuped SST files for destination TiKV cluster, default to the source TiKV cluster. Available options: "v1", "v1ttl", "v2".
      --end string               The end key of the backup task, key is exclusive.
      --format string            The format of start and end key. Available options: "raw", "escaped", "hex". (default "hex")
  -h, --help                     help for raw
      --start string             The start key of the backup task, key is inclusive.

...

[pingyu]

Rest LGTM~

[zeminzhou]

LGTM~

[pingyu]

LGTM~