feat: promiseify

[dead-horse]

No description provided.

[mention-bot]

@dead-horse, thanks for your PR! By analyzing the history of the files in this pull request, we identified @fengmk2, @csbun and @fangk to be potential reviewers.

[dead-horse]

#19 有问题，这个全部 promise 化了，这个合并后发 3.0.0，我会用 2.6.1 的代码再发布一次 2.7.1，deprecate 掉 2.7.0

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@60c3f18). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #20   +/-   ##
=========================================
  Coverage          ?   98.91%           
=========================================
  Files             ?        6           
  Lines             ?      277           
  Branches          ?       40           
=========================================
  Hits              ?      274           
  Misses            ?        3           
  Partials          ?        0

Impacted Files	Coverage Δ
lib/client.js	95.23% <100%> (ø)
lib/operator.js	100% <100%> (ø)
lib/literals.js	100% <100%> (ø)
lib/transaction.js	100% <100%> (ø)
lib/connection.js	100% <100%> (ø)

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[dead-horse]

�先合并发 3.0

[AntiMoron]

promisify可能会影响了prototype下继承链内的this

await db.beginTransactionScope(async (connection, ctx) => {
  const { query, commit } = connection;
  const ret = await query('select now() as currentTime;'); // <-- 这里会在底层实现里找不到this
  console.log(ret);
  await commit(); // <-- 这里会在底层实现里找不到this
  return { success: true };
});