Clean up execution context

[acquamarin]

Removes the following fileds from executionContext:

    uint64_t numThreads;
    storage::MemoryManager* memoryManager;
    storage::BufferManager* bufferManager;
    common::VirtualFileSystem* vfs;
    main::Database* database;

Saves the database inside the clientContext.

[codecov]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (8ecc270) 93.44% compared to head (0eeabf8) 93.44%.
Report is 2 commits behind head on master.

Files	Patch %	Lines
src/common/file_system/file_system.cpp	0.00%	6 Missing ⚠️
src/common/file_system/virtual_file_system.cpp	57.14%	6 Missing ⚠️
src/main/client_context.cpp	69.23%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2724      +/-   ##
==========================================
- Coverage   93.44%   93.44%   -0.01%     
==========================================
  Files        1073     1072       -1     
  Lines       40526    40544      +18     
==========================================
+ Hits        37869    37885      +16     
- Misses       2657     2659       +2     

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