compression/appendToBlockAndRotate-bug-fix: fix a panic caused by under-allocated write buffer

[bom-d-van]

The bug was noticed in a production whisper file with a stacktrace attached bellow.

Essentially, it's due to MaxCompressedPointSize size being under-estimated by 0.125 bits and caused an
off-by-1 panics in *archiveInfo.AppendPointsToBlock. Instead of increasing the MaxCompressedPointSize
const value, I decided to go with a more generous buffer allocation in *archiveInfo.appendToBlockAndRotate.

More explanations are in the comment of the changes.

panic: update_many_for_archive panics: runtime error: slice bounds out of range [:20] with capacity 19
goroutine 1 [running]:
runtime/debug.Stack()
	/opt/homebrew/Cellar/go/1.18.1/libexec/src/runtime/debug/stack.go:24 +0x68
github.com/go-graphite/go-whisper.(*Whisper).UpdateManyForArchive.func1()
	~/go/src/github.com/go-graphite/go-whisper/whisper.go:901 +0x40
panic({0x1010b1260, 0x1400014e060})
	/opt/homebrew/Cellar/go/1.18.1/libexec/src/runtime/panic.go:838 +0x204
github.com/go-graphite/go-whisper.(*archiveInfo).AppendPointsToBlock.func1()
	~/go/src/github.com/go-graphite/go-whisper/compress.go:888 +0x390
github.com/go-graphite/go-whisper.(*archiveInfo).AppendPointsToBlock(0x1400014c120, {0x1400014e030, 0x13, 0x0?}, {0x14000160000, 0x1, 0x3c})
	~/go/src/github.com/go-graphite/go-whisper/compress.go:1108 +0x264
github.com/go-graphite/go-whisper.(*archiveInfo).appendToBlockAndRotate(0x1400014c120, {0x14000160000, 0x1, 0x3c})
	~/go/src/github.com/go-graphite/go-whisper/compress.go:606 +0xc4
github.com/go-graphite/go-whisper.(*Whisper).archiveUpdateManyCompressed(0x14000150000, 0x1400014c120, {0x14000124028?, 0x10106d458?, 0x10109e660?})
	~/go/src/github.com/go-graphite/go-whisper/compress.go:537 +0x4b4
github.com/go-graphite/go-whisper.(*Whisper).UpdateManyForArchive(0x14000150000, {0x14000124028?, 0x1, 0x1}, 0xffffffffffffffff)
	~/go/src/github.com/go-graphite/go-whisper/whisper.go:943 +0x2a8
github.com/go-graphite/go-whisper.(*Whisper).UpdateMany(...)
	~/go/src/github.com/go-graphite/go-whisper/whisper.go:871
main.main()
	~/go/src/github.com/go-graphite/go-whisper/cmd/write.go:106 +0x748

goroutine 1 [running]:
main.main()
	~/go/src/github.com/go-graphite/go-whisper/cmd/write.go:107 +0x7d8
exit status 2