Upgrade to lucene-7.0.0-snapshot-ad2cb77. (#25349)

Most notable changes:
 - better update concurrency: LUCENE-7868
 - TopDocs.totalHits is now a long: LUCENE-7872
 - QueryBuilder does not remove the boolean query around multi-term synonyms:
   LUCENE-7878
 - removal of Fields: LUCENE-7500

For the `TopDocs.totalHits` change, this PR relies on the fact that the encoding
of vInts and vLongs are compatible: you can write and read with any of them as
long as the value can be represented by a positive int.

Author: jpountz

buildSrc/version.properties

@@ -1,6 +1,6 @@
 # When updating elasticsearch, please update 'rest' version in core/src/main/resources/org/elasticsearch/bootstrap/test-framework.policy
 elasticsearch     = 6.0.0-alpha3
-lucene            = 7.0.0-snapshot-92b1783
+lucene            = 7.0.0-snapshot-ad2cb77
 
 # optional dependencies
 spatial4j         = 0.6

core/licenses/lucene-analyzers-common-7.0.0-snapshot-92b1783.jar.sha1

@@ -0,0 +1 @@
+00d3260223eac0405a82eeeb8439de0e5eb5f888

core/licenses/lucene-analyzers-common-7.0.0-snapshot-ad2cb77.jar.sha1

@@ -0,0 +1 @@
+3a698989219afd9150738899bc849075c102881b

core/licenses/lucene-backward-codecs-7.0.0-snapshot-92b1783.jar.sha1

@@ -0,0 +1 @@
+bb636d31949418943454dbe2d72b9b66cd743f9f

core/licenses/lucene-backward-codecs-7.0.0-snapshot-ad2cb77.jar.sha1

@@ -0,0 +1 @@
+720252d786273edcc48b2ae7b380bc229fe8930c