bucket.Put: copy key before seek

[fuweid]

Application might change key value after seeking and before real put.
This unexpected behaviour could corrupt database. When users file issue,
maintainers doesn't know application behaviour. It could be caused by
data race. This patch is to prevent such case and save maintainers' time.

REF: #72

[ahrtr]

It doesn't necessarily cause elements to point freed pages. In this test case, if you change the key to a larger key e.g. 200, the case will pass.

Suggested change

	// When application inserts key/value, that key can be changed by application
	// during insertion. bbolt should copy key before seeking and updating. Otherwise,
	// some branch node could point to freed page.
	// When bbolt is processing a `Put` invocation, the key might be concurrently
	// updated by the application which calls the `Put` API (although it shouldn't).
	// It might lead to a situation that bbolt use an old key to find a proper
	// position to insert the key/value pair, but actually inserts a new key.
	// Eventually it might break the rule that all keys should be sorted. In a
	// worse case, it might cause page elements to point to already freed pages.

[fuweid]

Thanks for the comment. Updated

[ahrtr]

lgtm

Thanks @fuweid

Please backport the PR to release-1.3 as well.