remove unused class CAddrMan; from db.h
Univalue: Do not build JSON escape list at runtime

No need to waste startup time building something that can be done
at compile time.

This also resolves a clang++ warning originally reported in #4714,
univalue/univalue_write.cpp:33:12: warning: array subscript is of type 'char
escapes['"'] = "\\"";
^~~~

etc.
build: add option for reducing exports (v2)

This was committed previously as 4975ae172 and reverted, because the flags were
applied even if the checks didn't pass. This is the same commit, fixed up to
actually disable the functionality when necessary.

Enabled automatically if boost >= 1.49.
See: https://svn.boost.org/trac/boost/ticket/2309

Also, check for a default visibility attribute, so that we can mark future
api functions correctly.
Merge branch '2014_univalue_warn' into merge-PR4715

Merge pull request #4715
Merge pull request #4725

e432a5f build: add option for reducing exports (v2) (Cory Fields)
Merge pull request #4672

22b3c4b remove unused class CAddrMan; from db.h (Philip Kaufmann)