Similarly as with kvdb.View this commits adds a reset closure to the
kvdb.Update call in order to be able to reset external state if the
underlying db backend needs to retry the transaction.
This commit adds a reset() closure to the kvdb.View function which will
be called before each retry (including the first) of the view
transaction. The reset() closure can be used to reset external state
(eg slices or maps) where the view closure puts intermediate results.
This commit changes the key derivation algo we use to emulate buckets
similar to bbolt. The issue with prefixing keys with either a bucket or
a value prefix is that the cursor couldn't effectively iterate trough
all keys in a bucket, as it skipped the bucket keys.
While there are multiple ways to fix that issue (eg. two pointers,
iterating value keys then bucket keys, etc), the cleanest is to instead
of prefixes in keys we use a postfix indicating whether a key is a
bucket or a value. This also simplifies all operations where we
(recursively) iterate a bucket and is equivalent with the prefixing key
derivation with the addition that bucket and value keys are now
continous.
This commit separates all etcd related sources (sans a few stubs and
config) from the rest of the source tree and makes compilation conditional
depending on whether the kvdb_etcd build tag is specified.
