Increase BEFORE_PAGING_CONSTANT and enable users to disable paging

[tafsiri]

See #672 for context.

We should increase our constant that sets the paging threshold and probably also allow users to override it or disable it. Another idea I discussed with @nsthorat would be whether we could detect that a single tensor was larger than this threshold and disable paging if that occurs.

[nsthorat]

We increased the value and we now allow users to override it with ENV.set('WEBGL_NUM_MB_BEFORE_PAGING', 1000);