Review use of sequences (db, balance)

[karmacoma]

Review use of sequences (db, balance) and drop use of them where not needed.

See: https://github.com/LiskHQ/lisk/blob/development/helpers/sequence.js

Belongs to: #449

[Tschakki]

As discussed, this issue is delayed for later, after 1.0.0 release.

[4miners]

Issue is still valid, sequences should be reviewed as follows:
We currently use 3 sequences (sequence, dbSequence, balancesSequence)

• Check if we need both sequence and dbSequence
• Check if usage is right - for example we shouldn't mix receive blocks with other API calls, as node can not receive blocks in time because of sequence is busy with other, not that important processing
• Review if we need sequences for http API calls at all
• Review if all processing of balances-related things (transactions) is performed through balancesSequence

[vitaly-t]

In case this is useful, there is a promise-based sequence method within the spex library that's used by pg-promise internally. And that library's root can be accessed via pgp.spex, so no need to add any extra dependencies.

And if somebody decides to use, I will, of course help ;)

[vitaly-t]

Quickly threw in a PR to make that spex module accessible: #1587

[karmacoma]

dbSequence can be removed. This is an odd artifact from early days of Lisk.

I think we should also rename other sequences to something like: blockSequence, poolSequence

Making sure use them sparingly and at the highest level in call stack, so that we do not run into a problem of co-dependent nested sequences.

[MaciejBaj]

dbSequence had been removed - #1715,
balancesSequence has been reviewed and the usages revised - #1730.