Implement Block GET/PUT with new schema

- `Arc`+`Mutex` is replaced by `parking_lot::RwLock,` decoupled
  Read+Write and ability to upgrade read locks into write locks if
  needed
- Schema has changed, `Db` is now a struct that implements `new()` to
  return a new instance of itself, pros/cons listed in code but tl;dr
  blockchain and pending transactions are separate now
- `custom_filters` now supports extracting Block json and Transaction
  json in separate functions too
- /block GET and PUT implemented, `Blocks` currently have one check
  (transactions appear in pending transaction)
- debug is working after something, dunno how I fixed it

1 files changed, 5 insertions, 1 deletions

diff --git a/TODO.md b/TODO.md
index 76292d2..4e65094 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,4 +1,7 @@
 # TODO
+## Process
+- [ ] we need our own representation of students and their grades, "there is no blockchain"
+
 ## Proof-of-work
 - [ ] pick a block proposal scheme (= pick hash function) [list of hash functions](https://en.bitcoinwiki.org/wiki/List_of_hash_functions)
 - [ ] check the nonce for incoming blocks
@@ -7,4 +10,5 @@
 - [ ] pick a user authentication scheme
     - [ ] implement it
 
-- [ ] Switch to RwLock (parking_lot)
+## Done & Brag
+- [x] Switch to RwLock (parking_lot) (done at 2021-04-07 03:43, two possible schemes to represent inner Db (ledger) in code)