修复单条查询时连接泄露问题

[hookokoko]

在ecron中是用eorm获取单条数据时，在task续约的时候会产生大量单结果查询，当超过150个select之后出现：

&mysql.MySQLError{Number:0x410, Message:"Too many connections"}

这种就是连接泄露了。看了下源码，只要在执行完for rows.Next() 遍历之后，这个连接就会被关闭，这里是Next源码：

func (rs *Rows) Next() bool {
    var doClose, ok bool
    withLock(rs.closemu.RLocker(), func() {
        // 这里如果读到最后一个数据，doClose为true，
        // 此时才会关闭，如果for Next()时数据没有遍历完就中断了，
        // 则就不会执行，那么这个连接就泄露了。
        doClose, ok = rs.nextLocked()
    })
    if doClose {
        rs.Close()
    }
    return ok
}

所以，改进就是在执行完 rows, err := sess.queryContext(ctx, qc.q.SQL, qc.q.Args...) 之后调用下rows.Close()

[codecov[bot]]

Codecov Report

Merging #188 (bc3a0ef) into dev (a97835a) will increase coverage by 0.01%. The diff coverage is 100.00%.

:exclamation: Current head bc3a0ef differs from pull request most recent head 446b1bf. Consider uploading reports for the commit 446b1bf to get more accurate results

@@            Coverage Diff             @@
##              dev     #188      +/-   ##
==========================================
+ Coverage   81.82%   81.83%   +0.01%     
==========================================
  Files          33       33              
  Lines        2657     2659       +2     
==========================================
+ Hits         2174     2176       +2     
  Misses        404      404              
  Partials       79       79              

Impacted Files	Coverage Δ
core.go	95.31% <100.00%> (+0.15%)	:arrow_up:

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