refactor(indexes): Create a generic TxGroupIndex and uses it on AddressIndex

[msbrogli]

Acceptance Criteria

1. Create a TxGroupIndex, which is an abstract class to group transactions according to a key. It will be used in future PRs.
2. Implemente MemoryTxGroupIndex and RocksDBTxGroupIndex, which are abstract classes as well.
3. Refactor AddressIndex to use the TxGroupItem. Specifically, MemoryAddressIndex must use MemoryTxGroupIndex, and RocksDBAddressIndex must use RocksDBTxGroupIndex.

[codecov]

Codecov Report

Merging #524 (ac3cef4) into dev (5b70281) will increase coverage by 0.02%.
The diff coverage is 98.83%.

@@            Coverage Diff             @@
##              dev     #524      +/-   ##
==========================================
+ Coverage   82.85%   82.87%   +0.02%     
==========================================
  Files         198      201       +3     
  Lines       18143    18205      +62     
  Branches     2564     2563       -1     
==========================================
+ Hits        15032    15088      +56     
- Misses       2549     2552       +3     
- Partials      562      565       +3     

Impacted Files	Coverage Δ
hathor/indexes/rocksdb_tx_group_index.py	97.43% <97.43%> (ø)
hathor/indexes/address_index.py	92.30% <100.00%> (-0.80%)	⬇️
hathor/indexes/memory_address_index.py	96.00% <100.00%> (-1.73%)	⬇️
hathor/indexes/memory_tx_group_index.py	100.00% <100.00%> (ø)
hathor/indexes/rocksdb_address_index.py	96.96% <100.00%> (+0.26%)	⬆️
hathor/indexes/tx_group_index.py	100.00% <100.00%> (ø)
hathor/p2p/node_sync.py	83.98% <0.00%> (-1.46%)	⬇️
hathor/wallet/base_wallet.py	96.06% <0.00%> (+0.19%)	⬆️

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