go/loader: limit concurrency of I/O operations during parsing
go/loader: limit concurrency of I/O operations during parsing

This is a putative fix for the file descriptor exhaustion problem
described in https://github.com/golang/go/issues/10306.

Change-Id: If603fb9bbaec1b53f6b44d15b2c202e4670035ce
Reviewed-on: https://go-review.googlesource.com/8421
Reviewed-by: Matt Joiner <a8e93ab406eacf05a57eca4b21bf179356365dd3@gmail.com>
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

josharian/tools,argandas/tools,golang/tools,cw2018/tools,nicot/tools,cw2018/tools,jdhenke/tools,matryer/tools,matryer/tools,argandas/tools,springning/tools,dmitris/tools,springning/tools,nicot/tools,matryer/tools,golang/tools,sshetty10/tools,runcom/tools,nicot/tools,syreclabs/go-tools,jdhenke/tools,syreclabs/go-tools,josharian/tools,sshetty10/tools,magiccao/tools,cmars/tools,nicot/tools,glasser/golang-tools,cmars/tools,springning/tools,josharian/tools,runcom/tools,cmars/tools,syreclabs/go-tools,magiccao/tools,glasser/golang-tools,runcom/tools,magiccao/tools,golang/tools,sshetty10/tools,sshetty10/tools,syreclabs/go-tools,glasser/golang-tools,dmitris/tools,cw2018/tools,runcom/tools,golang/tools,jdhenke/tools,magiccao/tools,jdhenke/tools,syreclabs/go-tools,glasser/golang-tools,syreclabs/go-tools,cmars/tools,dmitris/tools,dmitris/tools,argandas/tools,springning/tools,argandas/tools,matryer/tools,cw2018/tools,golang/tools,josharian/tools