[FTS] stats: FTS RAM used stat goes over allocation and doesn't match RSS

Build : 4.7.0-1308

Steps :
1. Have a KV+FTS node with 256 MB allocated to FTS and the remaining (or whatever) is allocated to KV.
2. Create a bucket and create an FTS index (maybe a default one). I used the travel-sample bucket that has 31K keys.
3. Start loading in data, and watch the index build up gradually.
4. While the index is building up, keep an eye on the FTS RAM used stat.

Issue 1: It is observed that at some point, the used FTS RAM goes beyond the allocated 256 MB. In my case, I saw it go upto 342 MB

Issue 2: It looks like the 'FTS RAM used' stat and the resident memory consumed by cbft (as shown in top on the node) are not in sync. While the stat was showing ~300 MB, when I did top, cbft was consuming 545 MB resident memory.

Pls see the attached screenshots for both these issues.