To do after #4543 is landed (opening a separate issue to reduce the scope of this PR as per discussion with @graydon):

• Audit gRandomEngine multi-threaded access
• Rename CatchupManager -> ApplyExternalizedLedgersManager
• Enforce that only one thread can reference an active LedgerTxn at all times. Add comment to LedgerTxn class saying it can only be used at apply time, everyone else should resort to read-only LedgerSnapshot class.
• Remove LM from valueExternalized flow in Herder. Herder should go directly to LedgerApplyManager, since it is the module that handles applying now.