Adds an helper class to shard mspdbsrv.

Currently there's a single instance of mspdbsrv shared between all the linkers. This can cause some OOMs if too many linkers are accessing it in parallel.

There's an undocumented environment variable which allows to specify the mspdbsrv endpoint to bound to a linker: _MSPDBSRV_ENDPOINT_ , by setting this variable to a unique value per target you ensure that one different instance of mspdbsrv will be used for every target.

Currently I only use this value for the .exe targets, I keep the shared one for the other ones.

This is hidden behind a 'GYP_USE_SEPARATE_MSPDBSRV' environment variable.

Patch by sebmarchand@chromium.org

R=scottmg@chromium.org, siggi@chromium.org

Review URL: https://codereview.chromium.org/83803003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1801 78cadc50-ecff-11dd-a971-7dbc132099af