verify large selections work for reads and writes

[jreadey]

Previously HSDS had a limitation of 100mb for any GET or POST request. This shouldn't be a factor since streaming support was added to HSDS. Check this out by creating a test to write then read back a large selection.

[mattjala]

I tested it - the REST VOL works fine uploading and downloading gigabytes of data after #26

[ron-kuhn]

@mattjala, latest build fails with compiler error rest_vol.c line 1270. Changed to: memcpy(buffer, (const char*)uinfo->buffer + uinfo->bytes_sent, data_size);