antispam_mem_table_size and antispam_base_table_size cannot be lowerd once initialized

[jiemingwang]

Description

antispam_mem_table_size & antispam_base_table_size control the max size of Badger table (or max size of individual .sst files). Once these parameters are initialzed, lowering them can cause POSIX process to fail and remain stuck, typically with an error similar to: “Arena too small, toWrite:40 newTotal:174483044 limit:174483027”.

For now, a note should be added to document the bebavoir.