Preliminary implementation of EntityIdMap

Change-Id: I9184530ea191e48b42fbf260847a584c65aae113
Added exception to EntityIdMap

Change-Id: I82aabe67506b097984c9a5aef9016c3c244e22e5
Take a InternalEntityIdFinder in ClaimInserter

Change-Id: I8d1eaa08a29b6e42634e7260662f4d721b55ff78
Some implementation in ClaimInserter

Change-Id: Idf1a918581e7d2657d5dfde037287c058baeb4c6
Move classes into ClaimStore namespace

Change-Id: If2892f99afbea5298a51eb780abfe2a97a14b57b
Split off ClaimRowBuilder to its own file and added tests

Change-Id: I07c69031b6e9219bda83da6532cd214ae387b6a4
Changed QueryEngine test namespace to Wikibase\Tests\QueryEngine

Change-Id: I2fee9059dd0592c1b44fa36a0bd6dc6cbb831f0f