Merge #2532

2532: Fix laziness while streaming blocks r=mrBliss a=kderme

Currently we noticed a big space leak, where ghc profiling indicated a peak of >2GB PINNED memory used and ps/top reported >4GB memory used. 

![img1](https://user-images.githubusercontent.com/11467473/90975548-d951bd80-e53d-11ea-96c5-4809c0cee53e.png)


Checking with noUnexpectedThunks in the `LedgerDB` this was reported:

```
"ExtLedgerState (HardForkBlock (': * ByronBlock (': * (ShelleyBlock TPraosStandardCrypto) ('[] *))))","LedgerDB"], 
unexpectedThunkCallStack = [("$dmnoUnexpectedThunks",SrcLoc {srcLocPackage = "ouroboros-consensus-0.1.0.0-inplace", 
srcLocModule = "Ouroboros.Consensus.Ledger.Extended", srcLocFile = "src/Ouroboros/Consensus/Ledger/Extended.hs", 
srcLocStartLine = 76, srcLocStartCol = 10, srcLocEndLine = 76, srcLocEndCol = 79}),("noUnexpectedThunks",SrcLoc 
{srcLocPackage = "cardano-prelude-0.1.0.0-inplace", srcLocModule = "Cardano.Prelude.GHC.Heap.NormalForm.Classy", 
srcLocFile = "src/Cardano/Prelude/GHC/Heap/NormalForm/Classy.hs", srcLocStartLine = 424, srcLocStartCol = 44, 
srcLocEndLine = 424, srcLocEndCol = 70})], unexpectedThunkClosure = Just "ThunkClosure {info = StgInfoTable {entry = 
Nothing, ptrs = 4, nptrs = 0, tipe = THUNK, srtlen = 81781936, code = Nothing}, ptrArgs = 
[0x000000420daeea98,0x000000420daeeb10,0x00000042119f8c68,0x000000420250cb38], dataArgs = []}"}

```

The leak appears when we open the ChainDB and stream blocks from the Immutable DB to the Ledger DB for validation. I noticed that the older the ledger snapshot we start, the bigger the memory peak.

What actually happens is that because of laziness the validation is basically split in two parts: 
- read every block from the db leading to a big memory peak and do all other actions like logging
- apply each block to the ledger db, which is a pure procedure.

Only after the ledger DB application starts, the gc is free to collect the blocks and the PINNED memory starts to decrease after the peak. This is more apparent on this graph

![img2](https://user-images.githubusercontent.com/11467473/90975697-4d409580-e53f-11ea-9952-7ebc06fb1595.png)

`compute`, which does all the ledger transitions fires up only after all PINNED memory is allocated.

The fix forces the ledger transitions to happen strictly. The new memory result:
![img3](https://user-images.githubusercontent.com/11467473/90975761-c5a75680-e53f-11ea-9c41-4844e8b2f4b1.png)

The PINNED memory is quickly garbage collected and never goes above 12MB:
![img4](https://user-images.githubusercontent.com/11467473/90975812-2fbffb80-e540-11ea-9e0c-e7aa99ef93c1.png)

I'll come back with some more details on this.

Co-authored-by: kderme <k.dermenz@gmail.com>