add new RPC implementation file `rpcmisc.cpp`
Move `getinfo` from rpcnet to rpcmisc

`getinfo` is a general info method which shows information
from multiple sources, it doesn't belong in rpcnet.cpp or
any of the other current RPC implementation files.
Move `settxfee` from rpcblockchain to rpcwallet

`settxfee` only affects the wallet, not the block chain.
Move `nTransactionFee` from main.cpp to wallet.cpp

Transaction fee is only used by the wallet.
No need for it to be in main.cpp.
Move `validateaddress` from rpcwallet to rpcmisc

Enables it in --disable-wallet compiles.
Delimit wallet-using part using #ifdef ENABLE_WALLET.
Move `createmultisig` from rpcwallet to rpcmisc

Enables it in --disable-wallet compiles.
Move `verifymessage` from rpcwallet to rpcmisc

Enables it in --disable-wallet compiles.
Remove ui_interface ThreadSafeAskFee residue

Missed these in ca2c83d (#3415).
Merge pull request #3412

c3a7f51 Move `verifymessage` from rpcwallet to rpcmisc (Wladimir J. van der Laan)
723a03d Move `createmultisig` from rpcwallet to rpcmisc (Wladimir J. van der Laan)
452955f Move `validateaddress` from rpcwallet to rpcmisc (Wladimir J. van der Laan)
cd7fa8b Move `nTransactionFee` from main.cpp to wallet.cpp (Wladimir J. van der Laan)
a943bde Move `settxfee` from rpcblockchain to rpcwallet (Wladimir J. van der Laan)
16bc9aa Move `getinfo` from rpcnet to rpcmisc (Wladimir J. van der Laan)
652e156 add new RPC implementation file `rpcmisc.cpp` (Wladimir J. van der Laan)
Make bitcoin compile without wallet if "db_cxx.h" is not present

Moved includes of "db.h" into #ifdef ENABLE_WALLET blocks or remove
them.