Issue 1533: DbLedgerStorage does not persist ExplicitLAC: Information (Header) of TransientLedgerInfo is not persisted

[eolivelli]

Motivation

DbLedgerStorage does not persist "ExplicitLAC" on RocksDB, so when you restart a Bookie the information is lost.
This work can be considered a follow up of @reddycharan work at #1527

Changes

• persist ExplicitLAC on DBLedgerStorage
• save ExplicitLAC on RocksDB (it is an optional field, so this change is backward compatible)
• enable testExplicitLACIsPersisted test even for DBLedgerStorage
• on TransientLedgerInfo rewind the ByteBuf in order to be able to use it again while writing to RocksDB
• use computeIfAbsent in getOrAddLedgerInfo (code clean up, but code looked strange, probably subject to some possible race condition)

Master Issue: #1533

[eolivelli]

@reddycharan please take a look as this is a follow up of your work
@merlimat please take a look as you are more confident with DbLedgerStorage than me

@jiazhai this is the needed fix in order to fully leverage ExplicitLAC on the new API and on Pulsar

@sijie I am not sure if we have to change also tryReadLastAddConfirmedAsync and readLastAddConfirmedAndEntryAsync, those are part of the Long-Poll feature, currently long-poll still does not benefit from ExplicitLAC