Introducing HeadOptions.MemPostingsEnsureOrderConcurrency flag

[duricanikolic]

This PR introduces a new configuration flag in HeadOptions, MemPostingsEnsureOrderConcurrency. The purpose of this flag is to make the number of CPU cores used for sorting postings lists configurable. The default value of the new flag is 0, meaning that all available CPUs are used (GMAXPROCS(0)), like it was the case before this PR.

Moreover, this PR changes the signature of MemPostings.EnsureOrder() by introducing parameter numberOfConcurrentProcesses, representing the maximal number of CPU cores used for this operation. If it is less or equal to 0, GOMAXPROCS(0) is used. GOMAXPROCS(0) was the default before introducing this parameter.

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7 out of 16 committers have signed the CLA.

✅ beorn7
✅ csmarchbanks
✅ codesome
✅ bboreham
✅ colega
✅ duricanikolic
✅ roidelapluie
❌ bwplotka
❌ alanprot
❌ alexqyle
❌ AlisonB319
❌ SuperQ
❌ nidhey27
❌ prymitive
❌ hayk96
❌ soonping-amzn
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