XDCR - Responses are recycled prematurely

As part of performance improvements in MB-60981, responses were started to be recycled. This needs to relooked because:

• When we have different mutations for the same doc key in a xmem batch, we only "batchGet" once for all the mutations of the same key i.e. only one response instance for mutations with same doc key (but different unique-key).
• So when we talk about "unique-key" mapped to lookupResp, there can be multiple entries of the hashmap pointing to the same response.
• Therefore, we cannot recycle the response as soon as we read from the map or as soon as incoming mutation looses CR, because there might be another uniqueKey which is still mapped to the recycled response.