Now that Charan is working on making ExplicitLAC persistent I would like to
start the discussion about merging the reader side view of LAC (ExplicitLAC
+ Pibbybacked/regular LAC)

Major points:
- Now (4.8) ExplicitLAC is to be read with LedgerHandle#readExplicitLAC
- ExplicitLAC is stored on LedgerStorage
- Regular LAC is stored inside each entry
- We have to support Long Poll
- We have to give access to ExplicitLAC on the new API (but I don't want to
add a readExplicitLAC on the new API)
- ExplicitLAC on the write side is only a matter of configuring the
scheduler, no changes are to be done on application code

Desiderata (from me):

The reader only uses Long-Poll or ReadHandle#readLastAddConfirmed and
transparently it receives the most accurate information from ExplicitLAC
and PiggyBacked LAC

In order to achieve this goal we will have to work on many parts, this is
my list:
- add current ExplicitLAC on readResponses
- merge ExplicitLAC and regular LAC on client side
- support watches for Long-Poll on Bookie side, to be triggered by