Make the ServerParameterSet map non-leaky

Right now we leak the map of server parameters contained inside the ServerParametersSet class leaks memory on shutdown. As far as I can tell, it is a global and currently implicitly leaked somehow without ASAN complaining.

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Why this was a problem:

We want to run WiredTiger in unit tests with higher level code. The setup of the WiredTigerKVEngine (the wt storage engine) requires manually setting up a couple server parameters here.

However, ASAN catches the memory leak in unit tests: it appears to be more strict in unit test binaries than the mongod binary.

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Temporary solution added in SERVER-43058: we used __lsan_ignore_object on the server parameters set up in wiredtiger_init.cpp to quiet ASAN for the unit tests.