Fixes race conditions causing the following test failures:

```
=== RUN   TestUnconfirmedTxs
    TestUnconfirmedTxs: rpc_test.go:342:
                Error Trace:    rpc_test.go:342
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestUnconfirmedTxs
    TestUnconfirmedTxs: rpc_test.go:343:
                Error Trace:    rpc_test.go:343
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestUnconfirmedTxs
    TestUnconfirmedTxs: rpc_test.go:345:
                Error Trace:    rpc_test.go:345
                Error:          Not equal:
                                expected: types.Txs{types.Tx{0x39, 0x44, 0x4d, 0x6c, 0x4b, 0x66, 0x46, 0x78, 0x3d, 0x45, 0x33, 0x33, 0x68, 0x47, 0x6e, 0x79, 0x58}}
                                actual  : types.Txs(nil)

                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -1,4 +1,2 @@
                                -(types.Txs) (len=1) {
                                - (types.Tx) (len=17) Tx{39444D6C4B6646783D45333368476E7958}
                                -}
                                +(types.Txs) <nil>

                Test:           TestUnconfirmedTxs
    TestUnconfirmedTxs: rpc_test.go:342:
                Error Trace:    rpc_test.go:342
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestUnconfirmedTxs
    TestUnconfirmedTxs: rpc_test.go:343:
                Error Trace:    rpc_test.go:343
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestUnconfirmedTxs
    TestUnconfirmedTxs: rpc_test.go:345:
                Error Trace:    rpc_test.go:345
                Error:          Not equal:
                                expected: types.Txs{types.Tx{0x39, 0x44, 0x4d, 0x6c, 0x4b, 0x66, 0x46, 0x78, 0x3d, 0x45, 0x33, 0x33, 0x68, 0x47, 0x6e, 0x79, 0x58}}
                                actual  : types.Txs{}

                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -1,3 +1,2 @@
                                -(types.Txs) (len=1) {
                                - (types.Tx) (len=17) Tx{39444D6C4B6646783D45333368476E7958}
                                +(types.Txs) {
                                 }
                Test:           TestUnconfirmedTxs
--- FAIL: TestUnconfirmedTxs (0.20s)
=== RUN   TestNumUnconfirmedTxs
    TestNumUnconfirmedTxs: rpc_test.go:364:
                Error Trace:    rpc_test.go:364
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestNumUnconfirmedTxs
    TestNumUnconfirmedTxs: rpc_test.go:365:
                Error Trace:    rpc_test.go:365
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestNumUnconfirmedTxs
    TestNumUnconfirmedTxs: rpc_test.go:364:
                Error Trace:    rpc_test.go:364
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestNumUnconfirmedTxs
    TestNumUnconfirmedTxs: rpc_test.go:365:
                Error Trace:    rpc_test.go:365
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestNumUnconfirmedTxs
--- FAIL: TestNumUnconfirmedTxs (0.09s)
```