Convert time.Time to int64

[jwilder]

This change converts all the uses of time.Time to int64 within the TSM engine. This was an idea initially presented in #5522, but didn't make sense to implement until query engine and other changes had completed.

Since we mainly using timestamps as UnixNano values anyways, the overhead of using a time.Time everywhere ends up creating a lot of garbage for GC to work though. This change should help with GC and larger queries that process a lot of points.

A simple count query over 100M points improved from 53s to 38s with this change.

$ time influx -database stress -execute "select count(value) from cpu"
name: cpu
---------
time    count
0   1e+08


real    0m52.670s
user    0m0.006s
sys 0m0.006s

After:

$ time influx -database stress -execute "select count(value) from cpu"
name: cpu
---------
time    count
0   1e+08


real    0m38.107s
user    0m0.005s
sys 0m0.006s

• CHANGELOG.md updated
• Rebased/mergable
• Tests pass
• Sign CLA (if not already signed)

[jwilder]

@benbjohnson

[benbjohnson]

lgtm. 👏 That's a nice 14s drop in query time!

[julienmathevet]

👍 nice refactoring