Fix creating index in mysql.

[wjywbs]

MySQL does not support "CREATE INDEX IF NOT EXISTS".

[codecov]

Codecov Report

Merging #31 into dev will decrease coverage by 0.19%.
The diff coverage is 20.00%.

@@            Coverage Diff             @@
##              dev      #31      +/-   ##
==========================================
- Coverage   55.56%   55.36%   -0.20%     
==========================================
  Files          21       21              
  Lines        3387     3399      +12     
==========================================
  Hits         1882     1882              
- Misses       1173     1183      +10     
- Partials      332      334       +2     

Impacted Files	Coverage Δ
sql.go	74.52% <20.00%> (-1.34%)	⬇️

[foxcpp]

Thanks! Honest question though, how hard it would be to actually switch from MySQL to MariaDB? If my understanding is correct, MariaDB can act as a drop-in replacement.

[foxcpp]

P.S. If you are doing this PR to use MySQL with maddy, the change you made will make its way into pending 0.3 release that is currently being blocked by this issue: foxcpp/maddy#226

You can use it now by cloning maddy repo, running go get github.com/foxcpp/go-imap-sql@dev and then ./build.sh as usual.

[wjywbs]

That comes down to the preferences of the DBAs.

I have a fix for foxcpp/maddy#226 as well: emersion/go-msgauth#30