fix slice bounds out of range/maxAllocSize bugs

[sasha-s]

when accessing the node data we used to use cast to
*[maxAllocSize]byte, which breaks if we try to go across maxAllocSize boundary.

This leads to occasional panics.
Sample stacktrace:

panic: runtime error: slice bounds out of range

goroutine 1 [running]:
github.com/boltdb/bolt.(*node).write(0xc208010f50, 0xc27452a000)
    $GOPATH/src/github.com/boltdb/bolt/node.go:228 +0x5a5
github.com/boltdb/bolt.(*node).spill(0xc208010f50, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/node.go:364 +0x506
github.com/boltdb/bolt.(*node).spill(0xc208010700, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/node.go:336 +0x12d
github.com/boltdb/bolt.(*node).spill(0xc208010620, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/node.go:336 +0x12d
github.com/boltdb/bolt.(*Bucket).spill(0xc22b6ae880, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/bucket.go:535 +0x1c4
github.com/boltdb/bolt.(*Bucket).spill(0xc22b6ae840, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/bucket.go:502 +0xac2
github.com/boltdb/bolt.(*Bucket).spill(0xc22f4e2018, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/bucket.go:502 +0xac2
github.com/boltdb/bolt.(*Tx).Commit(0xc22f4e2000, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/tx.go:150 +0x1ee
github.com/boltdb/bolt.(*DB).Update(0xc2080e4000, 0xc24d077508, 0x0, 0x0)
    $GOPATH/src/github.com/boltdb/bolt/db.go:483 +0x169

It usually happens when working with large (50M/100M) values.
One way to reproduce it is to change maxAllocSize in bolt_amd64.go to 70000 and run the tests.
TestBucket_Put_Large crashes.

[sasha-s]

ping

[benbjohnson]

@sasha-s sorry for the delay getting back to you on this PR. I'm AFK until next Tuesday but I will look at this PR when I get back. Thanks for submitting the issue.

[benbjohnson]

@sasha-s This looks good to me. I pulled down the branch and did additional testing and tweaking to verify the changes. I also added some additional inline comments to bf5458d.
👍