Add garbage collector agg function

[ymarcon]

Add the possibility to trigger the R garbage collector on the server side by calling gc(), as a complement of the rm() function (called by rmDS).

> gc()
          used (Mb) gc trigger  (Mb) max used  (Mb)
Ncells 1695520 90.6    2607968 139.3  2607968 139.3
Vcells 3576174 27.3    8388608  64.0  4930149  37.7

Bonus is to return the output of gc: it is not disclosive, and it may be useful for the user.

This would encourage good memory management practices on the R server side, for a better usage of the shared system resources.