Summary:
Change the linking of tests/tools to be against a library rather than a list of objects.  This change substantially reduces the size of the objects produced.

peterd clean repo size: 264M
Before this change, with make all: 40G
After this change, with make all: 28G
With make LIB_MODE=shared all: 7.0G

The list of TESTS was changed from being hard-coded to generated from the test sources variable.  Note that there are some test sources that are not built as tests (though the set of tests is identical to the previous version).

Added OBJ_DIR option to Makefile to allow objects to be placed in an alternative location.  By default, OBJ_DIR is the same as before ("./").

This change is a precursor to being able to build/run the tests/tools linked against static libraries.  Additionally, it should be possible to clean up and merge some of the rules for building tests and the like if so desired.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6660

Reviewed By: riversand963

Differential Revision: D22244463

Pulled By: pdillinger

fbshipit-source-id: db9c6341d81ed62c2270374f4ede02fb9604c754