[YSQL] Analyze the memory usage of ysql postgres process

[lnguyen-yugabyte]

Jira Link: DB-3128

Description

When starting ysql, it consumes a significant amount of memory on yugabyte, even when starting fresh:

yugabyte=# select yb_mem_usage_kb();
 yb_mem_usage_kb
-----------------
           66604
(1 row)

On postgres, the equivalent takes less than 5 MB.

We'd want to break down and understand where the memory usage comes from, and apply necessary optimizations if possible.