Improve performance of batch operation serializer

[drobertduke]

This is a follow-up to #59, addressing the performance penalties of the code I wrote to enforce a certain order in batch operations, as discussed here: #59 (comment)

This replaces the worst-performing lodash and standard library calls. In micro-benchmarks, _startBatchIfReady runs in about half the time after these changes are applied.

[murgatroid99]

Can you rebase this change against the v1.10.x branch and either update this PR or send a new one?

[drobertduke]

@murgatroid99 The code this is based on is not in v1.10.x, #59 was merged to master after 1.10.x was cut.

[murgatroid99]

You're right. Never mind.