Warmup should continue after data loading based on access log

Under the current design, ep engine stops warmup (therefore data loading) and enable data traffic immediately after the data loading phase based on access log is complete.

This may not be an optimal way of completing warmup because the access log may not have enough key-value entries to fill up the memory upto the low-watermark. We should allow warmup continues in the background to fill up the memory even after the data loading based on access log is complete. However, enable data traffic immediately after the data loading though.