refactor(state): split the old state into state reader and state writer structs

[MaksymMalicki]

This PR introduces the following change. The current implementation of the State gets split into two structs:

• StateReader - a structure, which sole purpose is reading the state. It's fed with db.KeyValueReader to enforce read-only mode for the state. Eventually we want to feed it only with the db.Snapshot. It is going to be mainly used during the RPC requests.
• State - a structure which purpose is reading and writing. The StateReader is a part of it. It is used during the synchronisation and execution. Currently it runs using an IndexedBatch, but eventually we want to feed it with an IndexedBatch-like structure, which will contain a write layer and db.Snapshot for reads.

[codecov]

Codecov Report

❌ Patch coverage is 72.98387% with 67 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.96%. Comparing base (27e3453) to head (1abc350).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
core/state_reader.go	80.62%	18 Missing and 13 partials ⚠️
core/state.go	54.00%	8 Missing and 15 partials ⚠️
core/trie/proof.go	0.00%	4 Missing and 3 partials ⚠️
core/pending_state.go	0.00%	2 Missing ⚠️
core/trie/trie.go	84.61%	0 Missing and 2 partials ⚠️
core/contract.go	88.88%	0 Missing and 1 partial ⚠️
rpc/v8/storage.go	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3291      +/-   ##
==========================================
- Coverage   75.98%   74.96%   -1.03%     
==========================================
  Files         377      378       +1     
  Lines       34614    34950     +336     
==========================================
- Hits        26301    26199     -102     
- Misses       6466     6719     +253     
- Partials     1847     2032     +185     

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