session: add current DB name to the general log

[jackysp]

What problem does this PR solve?

Add current DB name to the general log.

Check List

Tests

• Manual test (add detailed scripts or steps below)
  Enable the general log and test a SQL.

2019/02/17 10:29:51.584 session.go:1644: [info] [GENERAL_LOG] con:2 user:root@127.0.0.1 schema_ver:28 txn_start_ts:0 current_db:test, sql:show tables

Related changes

• Need to cherry-pick to the release branch

[codecov-io]

Codecov Report

Merging #9332 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #9332      +/-   ##
==========================================
- Coverage    67.2%   67.19%   -0.01%     
==========================================
  Files         371      371              
  Lines       77571    77572       +1     
==========================================
- Hits        52131    52126       -5     
- Misses      20783    20788       +5     
- Partials     4657     4658       +1

Impacted Files	Coverage Δ
session/session.go	72.63% <0%> (-0.08%)	⬇️
util/systimemon/systime_mon.go	80% <0%> (-20%)	⬇️
infoschema/infoschema.go	76.31% <0%> (-1.32%)	⬇️
executor/join.go	78.38% <0%> (-0.53%)	⬇️
executor/executor.go	67.03% <0%> (+0.13%)	⬆️
expression/schema.go	94.53% <0%> (+0.78%)	⬆️

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[XuHuaiyu]

LGTM

[alivxxx]

LGTM

[lysu]

/run-all-tests