build: Improve handling of hiredis and zstd dependencies

build: Improve handling of hiredis and zstd dependencies

Introduced a new CMake variable called DEPS with the following
semantics:

- AUTO (the default): Use dependencies from the local system if
  available. Otherwise: Use bundled dependencies if available.
  Otherwise: Download dependencies from the internet (dependencies will
  then be linked statically).
- DOWNLOAD: Use bundled dependencies if available. Otherwise: Download
  recommended versions from the internet (dependencies will then be
  linked statically).
- LOCAL: Use dependencies from the local system if available. Otherwise:
  Use bundled dependencies if available.

The old {HIREDIS,ZSTD}_FROM_INTERNET and OFFLINE variables have been
removed in favor of DEPS.

Also streamlined the CMake code for hiredis and zstd.