Set up caching to speed up build and test.

[thetorpedodog]

This change unifies the build scripts between Mac OS and Linux, and caches build outputs. This means that future builds of the same version don’t need to rerun the entire build process; they can use the previously-built version.

We do a few things to accomplish this:

• Move compilation/installation into the ~/tiledb-core/install directory. This allows us to cache it.
• Set appropriate CGO_*FLAGS variables to point to that directory. This includes the rpath directive, which embeds the full path to the dynamic library in the binary, so it knows it needs to look in ~/tiledb-core/install/lib for libtiledb.so.
• Matrixes things out to reduce repetition.

This also fixes the ASAN step, which was never actually compiling.