Make leveldb options configurable

[JustinAzoff]

flow-indexer/store/leveldb.go

Lines 29 to 36 in 52b1dce

	opts.BlockSize = 32768
	opts.WriteBuffer = 67108864
	opts.BlockCacheCapacity = 524288000
	opts.OpenFilesCacheCapacity = 1024
	opts.CompactionTableSize = 32 * 1024 * 1024
	opts.WriteL0SlowdownTrigger = 16
	opts.WriteL0PauseTrigger = 64
	opts.Filter = filter.NewBloomFilter(10)

[JustinAzoff]

ah, one trickyness with this is there is support for more than one store, but I could add a somewhat generic store option type? some sort of cache size would be common to many of them.

[JustinAzoff]

also https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis

[JustinAzoff]

From testing, it seems the problem for memory usage is mostly the WriteBuffer, so I may just release a new version that drops that back to the default.