toku_db_set_* (the dbt setting functions)
no longer require atomicity for ENOMEM cases.
Also, in the case of realloc, we only alloc new memory if
the new size is larger than the old size, or the new size is less than
half of the old size.

Code has some simplifications due to lesser atomicity requirements.

Modified src/tests/test_db_dbt_mem_behavior.c to take into account
that DB_DBT_REALLOC now uses the ulen field.

git-svn-id: file:///svn/tokudb@4504 c7de825b-a66e-492c-adef-691d508d4ae1