PCP-24: Improve the performance of the HTTP API for getting all regions

[nolouch]

Description

There is a test that dumps all regions(about 100w) that need about 20s. the test way:

# create regions
create table x(a int);
split table x between (0) and (10000000) regions 100000;
split table x between (10000000) and (20000000) regions 100000;
# dump regions
time curl http://127.0.0.1:2379/pd/api/v1/regions > region.json
real	0m22.316s
user	0m0.063s
sys	0m0.948s
....
# some debug logs
[2019/10/23 12:19:55.653 +00:00] [INFO] [region.go:154] ["[DEBUG] got all regions from core"] [cost=149.863003ms]
[2019/10/23 12:19:59.083 +00:00] [INFO] [region.go:157] ["[DEBUG] convert all regions to API regions"] [cost=3.430113877s]
[2019/10/23 12:20:17.575 +00:00] [INFO] [region.go:160] ["[DEBUG] send all regions to client"] [cost=18.491626948s]

and a memory pprof:
mem.pprof.zip
alloc_space svg:
profile001.svg.zip

Goal

• Shorten request time
• Reduce memory allocation

Difficulty

• Medium

Score

• 900

Mentor(s)

• @nolouch

[ekalinin]

/pick-up-challenge

[sre-bot]

@ekalinin pick up issue success

[ekalinin]

/pick-up-challenge

[sre-bot]

@ekalinin pick up issue success