Add configs and metrics to transaction indexer

[magicxyyz]

Part of NIT-3330

This PR:

• add config for number of threads used in tx indexer to rlp decode blocks (previously hardcoded to runtime.NumCPU)
• adds configurable delay of tx indexer runs (minBatchDelay):
  • it mitigates the issue of tx indexer reading tx indexer tail from disk after each new head, what seemingly degraded node sync performance (experiments used pathdb, but probably applies also when hashdb is used)
  • delaying runs allows batching work for each run, so each run may be started for accumulated over the time block range
• adds new txindexer metrics:
  • txindexer/tail gauge - tracks oldest indexed block, useful e.g. for verifying if we keep up with unindexing
  • txindexer/unindexed/blocks/count and txindexer/unindexed/transactions/count - those expose stats about blocks / transactions unindexed in single txindexer run, useful to monitor how changing minBatchDelay affects actual work done in each batch

[MishkaRogachev]

Overall it looks good, just are a few questions

[MishkaRogachev]

LGTM

[tsahee]

LGTM