Fix memory leak of pfs keys

[prohaska7]

Valgrind finds memory leaks of pfs keys for some ydb tests. These tests create an environment, manipulate it, and close it. These tests do not open the environment. The bug is that the flag that controls the toku_ydb_destroy function is only set true if the environment is opened. The flag should be set in the toku_ydb_init function.

Reproduce: ctest -R ydb/test_cachesize -D ExperimentalMemCheck

Tools: Ubuntu 19.10, Clang 7,8,9, Gcc 7,8,9, Cmake 3.14

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