lightwalletd: Implement mechanism to allow log rotation (a la zcashd)

[LarryRuane]

If the log file is never trimmed, it can exhaust all available disk space. Zcashd has a mechanism to allow a tool like logrotate to do this. (Zcashd inherited this from Bitcoin Core.) It will be very easy to add this mechanism to lightwalletd, just a few lines of code.

The basic idea is the logrotate tool renames the existing log file and sends the process (lightwalletd) a SIGHUP signal, causing it to reopen the log file (and create it since it no longer exists with that name). The logrotate tool can then archive, compress, delete or whatever (according to how it's configured) the non-current log file.

[braddmiller]

This is a good idea, and probably a very small ticket, I'm going to move it way down the backlog but we will implement it if log storage becomes a problem.