add SqlGetChannelTips
add `History::GetChannelTips()`
TEST: `T_History::GetChannelTips()`
remove channel_tops_ from manifest
history API adaption in cvmfs.cc
add `History::GetHashes()`
TEST: `T_History::GetHashes()`
retrieve tag based on given date
TEST: `T_History::GetTagByDate`
API adaptions for new History
remove legacy history code