Various efficiency improvements for the query planner

[joka921]

These are rather minor changes in the code, but with a very significant effect for "large" queries (that is, queries with many operations):

1. Replace std::optional<const TripleGraph> by boost::optional<const TripleGraph&>; before this change, many copies of TripleGraphs were made for large queries; note that std::optional does not work for const &
2. Avoid re-building the cache key from scratch for each call to Operation::getCacheKey
3. Avoid re-computing the result width from scratch for each call to Operation::getResultWidth
4. We can now call setLimit for a whole QueryExecutionTree (before: only for an individual Operation); this invalidates the cache key and the result width
5. Make the variable name check (which calls the ANTLR parser) an expensive check, so that these are only executed in test and in debug mode

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.91%. Comparing base (acb6633) to head (4a91c4a).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1724      +/-   ##
==========================================
+ Coverage   89.86%   89.91%   +0.04%     
==========================================
  Files         389      393       +4     
  Lines       37308    37574     +266     
  Branches     4204     4224      +20     
==========================================
+ Hits        33527    33783     +256     
- Misses       2485     2495      +10     
  Partials     1296     1296              

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[sparql-conformance]

Conformance check passed ✅

No test result changes.

Details: https://qlever.cs.uni-freiburg.de/sparql-conformance-ui?cur=599aecd489c7e5bf78d1aa799de498aea0474664&prev=acb6633debc7341985341aff147b5038cc8d951b

[hannahbast]

Awesome, thanks a lot. Two minor changes (one is a suggestion)

[hannahbast]

Great!

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud