Move network-time related functions to timedata.cpp/h

The network time-offset-mangement functions from util.cpp are moved to
timedata.(cpp|h). This breaks the dependency of util on netbase.
Remove unnecessary dependencies for bitcoin-cli

This commit removes all the unnecessary dependencies (key, core,
netbase, sync, ...) from bitcoin-cli.

To do this it shards the chain parameters into BaseParams, which
contains just the RPC port and data directory (as used by utils and
bitcoin-cli) and Params, with the rest.
Move coins.cpp and keystore.cpp to libbitcoin_common

Prepare for introduction of `bitcoin-tx` tool.
Merge pull request #4368

75c82d4 Move coins.cpp and keystore.cpp to libbitcoin_common (Wladimir J. van der Laan)
84ce18c Remove unnecessary dependencies for bitcoin-cli (Wladimir J. van der Laan)
14f888c Move network-time related functions to timedata.cpp/h (Wladimir J. van der Laan)
Remove timing-based signature cache unit test

Two changes:

First removes a unit test that fails in my development environment
(OSX, compiled -g3 with clang).

sipa says that's not terribly surprising; the CMutableTransaction change
makes signing a little more expensive but verification quicker. The unit
test timed sign+verify-uncached versus verify-cached-five-times.

He also says the test will be invalid when libsec256kp1 is integrated
(because validation is super-optimized over signing).

core.h change fixes a compiler warning (clang -Wall : CMutableTransaction defined
as struct, declared as class in script.h).
JSON-RPC method: prioritisetransaction <txid> <priority delta> <priority tx fee>

Accepts the transaction into mined blocks at a higher (or lower) priority
Merge pull request #4414

a90689f Remove timing-based signature cache unit test (Gavin Andresen)
Merge pull request #1583

2a72d45 JSON-RPC method: prioritisetransaction <txid> <priority delta> <priority tx fee> (Luke Dashjr)
replace ComputeMinWork with CheckMinWork
Remove redundant pblock->nNonce = 0