support `INSERT IGNORE` with strict sql mode produces warning instead…

… of error (pingcap#4376)

* fix infoschema not support unsigned

parser: support DISTINCTROW(pingcap#4007)

json: fix unquote bug and add test cases. (pingcap#3507)

plan: remove useless aggregation when pushing down aggregation (pingc…

…ap#2702)

plan: only plans that have apply will add cache. (pingcap#2564)

kv: update logs (pingcap#2307)

plan: fix a panic bug. (pingcap#1914)

*: Use prometheus for metrics (pingcap#1525)

Add prometheus metrics framework.