Add a Compact() function to gtreap store.

[tanzhang0504]

This is a fix for memory leak from in-mem index (#374).

I made a fix in the underlying store - #1314. but seems like Compact() function is recommended by the owner - blevesearch/blevex#30.

[tanzhang0504]

I did not add batch_size as done by compact() in other two stores, because the in-memory operation is usually much faster, and the caller can adjust the frequency of calling compact() to limit the latency per compact(). PLMK your thoughts.