FindBoost: Overhaul caching and search repeat behavior

Overhaul the implementation as follows:

(1) Do not cache result variables such as Boost_VERSION,
Boost_LIB_VERSION, Boost_LIBRARY_DIRS, Boost_${COMPONENT}_FOUND,
Boost_${COMPONENT}_LIBRARY, or Boost_LIB_DIAGNOSTIC_DEFINITIONS that are
derived uniquely from other search results.  The user should not edit
them anyway.

(2) Add cache value Boost_LIBRARY_DIR to hold the single directory
expected to contain all libraries.  Once one library is found, search
only that directory for other libraries.

(3) Use the find_library NAMES_PER_DIR option to consider all possible
library names at the same time.

(4) Collect all documented input and cache variables and detect when
they have been changed by the user.  Discard prior search results that
may have been influenced by the changes and search for them again.
Environment variables are not expected to be persistent so use them only
as hints and do not consider changes to them to be meaningful.

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