Move CMedianFilter to timedata.cpp

Now that we no longer use the median filter to keep track of
the number of blocks of peers, that's the only place it is used.
Remove unused `alignup` function from util.h
Remove unused function `ByteReverse` from util.h
Move SetThreadPriority implementation to util.cpp instead of the header

Put the THREAD_* and PRIO_ constants in compat.h.
move functions in main and net to implementation files
Move functions in wallet.h to implementation file

Breaks compile-time dependency of wallet.h on util.
Move `S_I*` constants and `MSG_NOSIGNAL` to compat.h
Move `*Version()` functions to version.h/cpp
Move `COIN` and `CENT` to core.h

Eventually these should end up in `money.h` after monetary
amounts are typedef'ed, but at least they don't belong in `util.h`.
Split up util.cpp/h

Split up util.cpp/h into:

- string utilities (hex, base32, base64): no internal dependencies, no dependency on boost (apart from foreach)
- money utilities (parsesmoney, formatmoney)
- time utilities (gettime*, sleep, format date):
- and the rest (logging, argument parsing, config file parsing)

The latter is basically the environment and OS handling,
and is stripped of all utility functions, so we may want to
rename it to something else than util.cpp/h for clarity (Matt suggested
osinterface).

Breaks dependency of sha256.cpp on all the things pulled in by util.
Merge pull request #4748

ad49c25 Split up util.cpp/h (Wladimir J. van der Laan)
f841aa2 Move `COIN` and `CENT` to core.h (Wladimir J. van der Laan)
6e5fd00 Move `*Version()` functions to version.h/cpp (Wladimir J. van der Laan)
b4aa769 Move `S_I*` constants and `MSG_NOSIGNAL` to compat.h (Wladimir J. van der Laan)
af8297c Move functions in wallet.h to implementation file (Wladimir J. van der Laan)
651480c move functions in main and net to implementation files (Wladimir J. van der Laan)
610a8c0 Move SetThreadPriority implementation to util.cpp instead of the header (Wladimir J. van der Laan)
f780e65 Remove unused function `ByteReverse` from util.h (Wladimir J. van der Laan)
121d6ad Remove unused `alignup` function from util.h (Wladimir J. van der Laan)
d1e26d4 Move CMedianFilter to timedata.cpp (Wladimir J. van der Laan)