Make RPC connection persistent to improve high throughput RPC
access where necessary; uses keep alive and recreates connection
when it drops.
Restrict listtransaction calls to the required account, plus only
looks back 100 txs (assuming concurrent txs less than this), thus
greatly reducing the number of gettransaction calls over RPC.
Fixes bug in choose_sweep_orders (was not filtering out non-sw
orders).
Removes tickchainthread from tests, so no longer any threads used
even in tests; replaces with reactor task loop.
Temporarily removes test_wallets and test_segwit from build tests,
since they used blocking which only worked in threaded tests; these
tests must be rebuilt.
Adam Gibson