Support multiple indexes in CustomIndexer module

[SupernaviX]

Blocked by #375

Different indexes might be at different points on-chain.

Make sure that indexes receive the correct events when more than one index is defined (i.e. if index A is at height 1000 and index B is at height 1500, index A should be notified about TXs from block 1001 and index B should not.)

Make sure that if one index returns an error while indexing, other indexes continue to advance.