Memory optimization of static_variant

Peerplays operation is typedef of static_variant. As blockchain operations are used everywhere in the database it would be great to reduce the memory footprint of this structure as much as possible.

static_variant contains member variable storage that is a static array of characters which size is deduced from the largest type of a variant. This means every operation in the database is as large as the largest type + 8 bytes for _tag + some bytes for memory alignment.

Changing from static_variant to raw_array would greatly reduce the memory as some operations(like asset-create operation) size is high.

Ref:https://github.com/bitshares/bitshares-fc/pull/85

Estimate: 16 hours including unit testing with replays