fix(indexes): RocksDB UTXO index could miss some results

[jansegre]

Also included in the fix is a small change in the internal implementation of the RocksDB UTXO index for the internal iterator to only yield entries that match the relevant prefix, instead of having to do the filtering from outside the iterator.

Acceptance criteria

• the RocksDB implementation of UTXO index must correctly return the UTXO when the target amount is smaller than the smallest UTXO and it is the only UTXO for that address

[codecov]

Codecov Report

Merging #493 (e9d3cd5) into dev (38d4e3e) will increase coverage by 0.00%.
The diff coverage is 89.65%.

@@           Coverage Diff           @@
##              dev     #493   +/-   ##
=======================================
  Coverage   83.50%   83.51%           
=======================================
  Files         179      179           
  Lines       16330    16344   +14     
  Branches     2505     2502    -3     
=======================================
+ Hits        13636    13649   +13     
- Misses       2242     2244    +2     
+ Partials      452      451    -1     

Impacted Files	Coverage Δ
hathor/indexes/rocksdb_utxo_index.py	89.15% <89.65%> (+2.28%)	⬆️
hathor/p2p/manager.py	66.88% <0.00%> (-1.34%)	⬇️

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