Bugfix: Store last/next wallet resend times unique per CWallet object

[luke-jr]

As soon as we have multiple CWallet objects, this bug would cause them to interfere with each other rebroadcasting.

[sipa]

Any reason not to just call the variables nNextTime/nLastTime, or rename their usages to nNextRetry/nLastRetry? That reference looks like a hack to avoid code changes :)

[sipa]

ACK otherwise, by the way.

[BitcoinPullTester]

Automatic sanity-testing: PASSED, see http://jenkins.bluematt.me/pull-tester/1ee947d0573f8f0b5ae8c5efb27fae376e867187 for binaries and test log.
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[laanwj]

ACK (after squashing into one commit)

Edit: and I agree with @sipa on the use of references, please just rename the variables.

[jgarzik]

ACK, once @laanwj and @sipa comments applied

[luke-jr]

Rebased with requested refactor.

[BitcoinPullTester]

Automatic sanity-testing: PASSED, see http://jenkins.bluematt.me/pull-tester/203d1ae69b1b606c0216d0dc35de567c591720ae for binaries and test log.
This test script verifies pulls every time they are updated. It, however, dies sometimes and fails to test properly. If you are waiting on a test, please check timestamps to verify that the test.log is moving at http://jenkins.bluematt.me/pull-tester/current/
Contact BlueMatt on freenode if something looks broken.