Remove heap allocations when flushing the nodes or cached leaves

[cec489]

Address issue #195

[cec489]

Learning about golang "escape analysis". I tried a building with -gcflags="-m", and shows that my changes from calling "make" to declaring a var, are not helping. For example, func (m *NodesBackEnd) dbPut (line 74:80)

blockchain/utreexoio.go:74:7: leaking param content: m
blockchain/utreexoio.go:75:6: moved to heap: buf
blockchain/utreexoio.go:78:2: moved to heap: serialized

[cec489]

My other change, adding ClearMaps doesn't allocate to the heap:

blockchain/internal/utreexobackends/cachedleavesmap.go:126:7: leaking param content: ms
blockchain/internal/utreexobackends/cachedleavesmap.go:130:16: make([]map[utreexo.Hash]uint64, len(ms.maxEntries)) escapes to heap
blockchain/internal/utreexobackends/cachedleavesmap.go:132:20: make(map[utreexo.Hash]uint64, ms.maxEntries[i]) escapes to heap
blockchain/internal/utreexobackends/cachedleavesmap.go:139:7: leaking param content: ms
blockchain/internal/utreexobackends/cachedleavesmap.go:153:7: leaking param content: ms

[kcalvinalvin]

Ran memory profiling on 853aa68d vs current master. Verified that the memory profiling does show that there's less overhead for 853aa68d.

Before:

After: