[Bro] define a variable name based on string in bro

[Jan Grashöfer]

> So testing if an element was seen before using a bloom filter has the same
> complexity as checking if its in a table ?

It is a different tradeoff: The worst case for a hash table would be a
lookup that takes O(n). The "worst case" for a bloom filter would be a
match although the tested element was not added to the filter.

Have a look at https://en.wikipedia.org/wiki/Hash_table and
https://en.wikipedia.org/wiki/Bloom_filter for more details about these
data structures.

Jan