move HistoryDatabase to history_sql.{h|cc}
add basic implementation of history::History
TEST: basic unit test set for History
add `History::OpenWritable()`
add `History::Remove()`
remove `Sql::IsBusy()` dependency
assert success of `Sql::Init()`
TEST: unit tests for `History::Remove()`
add `History::SetPreviousRevision`
add `History::Exists()` and `History::Get()`
TEST: adapt history unit test cases
add `History::Tag::GetChannelName`
add SqlGetChannelTips
add `History::GetChannelTips()`
TEST: `T_History::GetChannelTips()`
add `History::GetHashes()`
TEST: `T_History::GetHashes()`
retrieve tag based on given date
TEST: `T_History::GetTagByDate`
remove legacy history code