On low-RAM systems, set Bitcoin Core to run with reduced memory

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Changes that will have little impact to performance:

1. After the IBD completes, reduce dbcache
2. Reduce the mempool with maxmempool=50
3. Possibly send a message to the user to avoid enabling listening mode
4. On Linux, set MALLOC_ARENA_MAX=1

[epiccurious]

Relates to #38

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This issue will not be part of the Minimum Viable Product milestone.