RAM cache usage growing non-stop past ram_cache.size

By leaving `proxy.config.cache.ram_cache.size` unspecified, trafficserver automatically determines the size of RAM cache. However, the computed limit does not seem to be enforced, and RAM cache usage keeps on growing well past the value.

On our production systems we have not specified a value for `proxy.config.cache.ram_cache.size`, and ATS assigned ~970M to `ram_cache.total_bytes`. We have however seen `bytes_used` going up to 2.5G counting.

```
$ traffic_ctl metric match .*cache.ram_cache.*bytes
proxy.process.cache.ram_cache.total_bytes 972832768
proxy.process.cache.ram_cache.bytes_used 2565339904
```
This very issue was already reported to the mailing list in the past: https://www.mail-archive.com/users%40trafficserver.apache.org/msg06809.html

Explicitly setting `proxy.config.cache.ram_cache.size` to `1G` works fine, cache memory usage does not grow past that value.