* common: delete StringToAddress, StringToHash
These functions are confusing because they don't parse hex, but use the
bytes of the string. This change removes them, replacing all uses of
StringToAddress(s) by BytesToAddress([]byte(s)).
* eth/filters: remove incorrect use of common.BytesToAddress
When implementing the new bloombits based filter, I've accidentally broke null
topics by removing the special casing of common.Hash{} filter rules, which
acted as the wildcard topic until now.
This PR fixes the regression, but instead of using the magic hash
common.Hash{} as the null wildcard, the PR reworks the code to handle nil
topics during parsing, converting a JSON null into nil []common.Hash topic.