feat: add safeguards for partially validated transactions [part 2/2]

[jansegre]

This change was split out from #577. The main objective is to be able to do this:

tx.set_validation(ValidationState.BASIC)  # this is a partially validated transaction
tx_storage.save_transaction(tx)  # this fails with an assertion error
with stx_storage.allow_partially_validated_context():
    tx_storage.save_transaction(tx)  # but this succeeds
tx_storage.get_transaction(tx.hash)  # this also fails, but with an error that inherits from "not found"
with stx_storage.allow_partially_validated_context():
    tx_storage.get_transaction(tx.hash)  # but this succeeds

This change is only exists to reduce the scope where the storage APIs accept reading/writing partially validated transactions so that the changes needed by sync-v2 can have a very reduced an limited impact on the rest of the node. The previous iteration of this protection was made using a allow_partially_validated: bool argument on most APIs, that ended up being not only verbose, but more error prone and needed to carry this allow_partially_validated in a lot of places that shouldn't have to know about that.

Acceptance Criteria

• TransactionStorage must have a scope that dictates which validation level on accept at save/get methods
• TransactionStorage must have methods that return contextmanagers which temporarily modify that scope

[codecov]

Codecov Report

Merging #617 (f7b7cd8) into master (17eacb2) will increase coverage by 0.05%.
The diff coverage is 94.23%.

❗ Current head f7b7cd8 differs from pull request most recent head 5005cb6. Consider uploading reports for the commit 5005cb6 to get more accurate results

@@            Coverage Diff             @@
##           master     #617      +/-   ##
==========================================
+ Coverage   83.78%   83.84%   +0.05%     
==========================================
  Files         238      237       -1     
  Lines       19990    20016      +26     
  Branches     2735     2731       -4     
==========================================
+ Hits        16749    16782      +33     
+ Misses       2642     2635       -7     
  Partials      599      599              

Impacted Files	Coverage Δ
hathor/manager.py	70.30% <71.42%> (-0.31%)	⬇️
hathor/transaction/base_transaction.py	91.23% <85.71%> (+1.24%)	⬆️
hathor/transaction/storage/tx_allow_scope.py	93.10% <93.10%> (ø)
hathor/consensus/consensus.py	100.00% <100.00%> (ø)
hathor/transaction/storage/cache_storage.py	94.16% <100.00%> (ø)
hathor/transaction/storage/exceptions.py	100.00% <100.00%> (ø)
hathor/transaction/storage/memory_storage.py	91.04% <100.00%> (ø)
hathor/transaction/storage/rocksdb_storage.py	94.77% <100.00%> (+2.07%)	⬆️
hathor/transaction/storage/transaction_storage.py	91.18% <100.00%> (+0.44%)	⬆️
hathor/transaction/transaction_metadata.py	90.90% <100.00%> (+1.17%)	⬆️

... and 11 files with indirect coverage changes