libpqxx 7.9.2: sundry fixes.

Here's what's new in this release:

• Fix CMake documentation install. (#848)
• Bump gcc/clang/postgres minimum version requirements.
• Another fix to the readthedocs documentation build. (#845)
• Remove obsolete CMake workaround kludges. (#851, #866, #867, #869)
• Remove obscure deprecated stream_to constructor that never worked. (#853)
• Support reading a field as an SQL array using as_sql_array(). (#841)
• Make row iterator a proper random_access_iterator. (#846)
• Downgrade result iterator to "bidirectional"; was never really RA. (#846)

You may wonder why result iterators are no longer random access iterators. Turns out they never were! That's because if you have a result iterator i, referring to a row, then i[n] in libpqxx refers to field n in that row. The definition for random access iterators expects it to mean *(i+n) instead.