perf(utils): improve getProp(), setProp(), delProp(), setGetter() performance

[dailyrandomphoto]

I found that on a huge site, the execution time of String.split() / extractPropKey() is very long.

function extractPropKey(key) {
  return key.split('.');
}

(https://github.com/hexojs/warehouse/blob/master/lib/util.js#L22-L24)

A benchmark using a hexo dummy website (with 300 posts): (clone https://github.com/SukkaLab/hexo-many-posts.git)

• extractPropKey() has been called for 7,201,086 times.

• For input values, they are all simple keys without "."

3325949  tag_id
2474477  post_id
 894848  _id
 178803  name
 131535  category_id
  17070  path
  16950  permalink
  16936  slug
  14418  length
  12964  date
  12490  posts
   9722  source
   7588  modified
   7468  hash
   6580  parent
   5084  published
   4460  updated
   4460  title
   4460  raw
   4460  layout
   4460  full_source
   4460  comments
   4460  _content
   4446  tags
   4446  photos
   4446  link
   4446  categories
   4446  asset_dir
   3820  more
   3820  excerpt
   3820  content
   3174  id
    120  renderable

A benchmark using a hexo dummy website (with 1500 posts): (clone https://github.com/SukkaLab/hexo-many-posts.git 5 times)

• Execution Time of extractPropKey(): 19.78s

Solution

This PR skips the call to split() and for-loops, handle simple keys directly.

[SukkaW]

Has it been tested in a dummy site?

[dailyrandomphoto]

@SukkaW
Here is data from test/benchmark.sh ran on Travis. (master, this pr)

A benchmark using a hexo dummy website (with 300 posts):

Node.js 8

	Total time (Cold)	Save DB (Cold)	Memory (Cold)	Total time (Hot)	Save DB (Hot)	Memory (Hot)
Current Master Branch	24.228s	0.611s	584.676MB	15.823s	0.667s	767.297MB
This PR	21.134s	0.562s	578.789MB	13.698s	0.557s	753.648MB

Node.js 10

	Total time (Cold)	Save DB (Cold)	Memory (Cold)	Total time (Hot)	Save DB (Hot)	Memory (Hot)
Current Master Branch	25.258s	0.780s	589.504MB	15.836s	0.777s	789.211MB
This PR	18.150s	0.576s	609.074MB	11.686s	0.591s	746.203MB

Node.js 12

	Total time (Cold)	Save DB (Cold)	Memory (Cold)	Total time (Hot)	Save DB (Hot)	Memory (Hot)
Current Master Branch	24.923s	0.784s	573.012MB	14.703s	0.783s	808.902MB
This PR	19.895s	0.596s	547.336MB	11.947s	0.712s	807.457MB

Node.js 13

	Total time (Cold)	Save DB (Cold)	Memory (Cold)	Total time (Hot)	Save DB (Hot)	Memory (Hot)
Current Master Branch	25.070s	0.707s	589.926MB	15.299s	0.716s	807.445MB
This PR	22.133s	0.620s	584.203MB	13.568s	0.717s	824.426MB