fix(store): caching account and validator in store

[b00f]

Description

The primary objective of this PR is to enhance the performance of the software and it also aims to reduce the syncing time. Executing each transaction requires fetching the corresponding account or validator information from LevelDB. Since database read/write operations are costly, they can significantly impact performance.

To mitigate this, we've implemented caching for accounts and validators based on their addresses. This approach allows us to write data into the DB, while reading account or validator information is performed from the cached data.

Since we are utilizing a map for caching, the complexity of fetching data is O(1), which ensures optimal performance. However, if the number of accounts becomes large, for example more than 1 million, we may need to use an LRU cache.

Another modification introduced in this PR is the implementation of the clone function for validators and accounts. Within both the sandbox and store packages, we assign objects by pointer and return a cloned object.

[Ja7ad]

@b00f All things is okay for merging.

[codecov]

Codecov Report

Merging #513 (2bf897e) into main (f550c79) will increase coverage by 0.09%.
The diff coverage is 95.61%.

❗ Current head 2bf897e differs from pull request most recent head 29d705f. Consider uploading reports for the commit 29d705f to get more accurate results

@@            Coverage Diff             @@
##             main     #513      +/-   ##
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+ Coverage   83.05%   83.15%   +0.09%     
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  Files         154      154              
  Lines        7313     7306       -7     
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+ Hits         6074     6075       +1     
+ Misses        955      951       -4     
+ Partials      284      280       -4     

[themantre]

Remove TestReturnNilForNonExistingItems test