Merge improvements from BitShares

[pmconrad]

... where applicable.

• Store correct block ID when switching forks bitshares/bitshares-core#938 - Store correct block ID when switching forks - Fix block storing #41
• Fix wallet in-code docs bitshares/bitshares-core#1015
• Add fail_reason to proposal_object bitshares/bitshares-core#1036 - add fail reason to proposal_object
• Short-cut long sequences of missed blocks bitshares/bitshares-core#1087 - short cut sequences of missed blocks
• remove needless find() in p2p bitshares/bitshares-core#1091
• add quit command to cli_wallet #1050 bitshares/bitshares-core#1104 - cli_wallet "quit" command
• Fix tests bitshares/bitshares-core#1126 - boost-1.66 compatibility - FC update #37
• remove double assert in object_id_type constructor bitshares/bitshares-core#1128 - double assert
• Fix compiler warnings bitshares/bitshares-core#1129
• Fix boost 1.66 compatibility bitshares/bitshares-core#1161 - boost-1.66 compatibility - FC update #37
• Suppressed compiler warnings in database_api_tests bitshares/bitshares-core#1181
• Fix 2 bugs reported by sonarqube bitshares/bitshares-core#1185 - minor fix
• Suppressed compiler warnings when generating wallet API docs bitshares/bitshares-core#1190
• CLI wallet: avoid directly overwriting wallet file on exit #1109 bitshares/bitshares-core#1195 - cli_wallet: avoid overwriting wallet file
• Suppressed compiler warning on wallet API docs bitshares/bitshares-core#1199
• Allow sufficient space for new undo_session bitshares/bitshares-core#1247
• Improve block generation performance bitshares/bitshares-core#1251 - block generation performance
• Fix proposal undo bitshares/bitshares-core#1257
• Changed block producing timeout to 2500 ms (#1157) bitshares/bitshares-core#1266 - fix node didn't wake up in time
• Change replay percentage to total block size processed #1289 bitshares/bitshares-core#1335 - replay percentage
• Performance opt pt 1 bitshares/bitshares-core#1359 - performance opt pt 1
• Terminate block production loop when shutting down witness plugin #1314 bitshares/bitshares-core#1364 - terminate block production when witness shuts down
• Build simplification bitshares/bitshares-core#1547 - build simplification - Ported build simplifications from BitShares #42
• Set Content-Type header in RPC responses #2869 steemit/steem#2871

[abitmore]

bitshares/bitshares-core#946 & bitshares/bitshares-core#1529 calling exit from libs / saving unclean db