Unique constraint on an index should throw an error if an item already exists

[slackpad]

This came up in hashicorp/consul#1463 where I found I could create multiple entries by name, even though there's a unique constraint on that index.

MemDB should throw an error if an item is being inserted into an index marked unique, and there's an existing item there, and that item's primary key doesn't match the incoming one.

[jmheidly]

I wonder why nobody has given this issue a thought. It is much needed fix as it defies the purpose of the unique property.

[armon]

@jmheidly Mostly because our apps that use it (Consul, Nomad) avoid this issue in higher level application code.

[zouqiang8233]

I found a problem. If the index uses the int type, not wrong, but can not query the results through the index.

[hbagdi]

#49 could solve this.