Clean up AccumulatorAvg

Major change is not inheriting from AccumulatorSum

Prep for SERVER-5044 $stdDev accumulator
SERVER-11216 Interrupted geoNear commands on mongos don't return an error code

Signed-off-by: Matt Kangas <matt.kangas@mongodb.com>
SERVER-11135 fixed appendNumber(long long) and appendIntOrLL to handle std::numeric_limits<long long>::min()
SERVER-11260 Remove all code related to advanced role delegation
SERVER-11203 Reverting changes to time support code to work with negative time_t

Doing this correctly depends on SERVER-11273

Revert "SERVER-11174 Reference db of tool test rather than global db object"
This reverts commit d1c2ae72c4829ed77cb6b7fe42e486910483caff.

Revert "SERVER-11174 Skip tests of negative dates on windows"
This reverts commit 9fcdfeb195c42e91a49e37212a65fea624d71c13.

Revert "SERVER-11174 Skip test for date that cannot be stored on 32 bit systems"
This reverts commit fc3b552e788e027af40523173790a20f49b27ec5.

Revert "SERVER-6058 Fix time_support_test on systems with small time_t"
This reverts commit c1a09966ff05c42c184870715e7a7383d02c477d.

Revert "SERVER-6058 Factor out date to UTC handling code and use in mongoexport"
This reverts commit 0b19be208f91e70c683d60c22011cad14a9fa78c.
SERVER-11135 Fix C++11 build break and questionable type narrowing.
SERVER-11281 Remove duplicate setting of default "admin" db
SERVER-11241: Updates allow creating $id/$ref/$db prefixed fields
SERVER-11241: remove unrelated stuff from prev commit
SERVER-11297: fix tailable cursor with non-existant collection
SERVER-7756: add test, was fixed by matcher refactor
SERVER-11141 Fixed SSL error message typo
SERVER-11025 Prevent adding users with username = server's subjectname
SERVER-8864 Tests for SSL mixed-mode
SERVER-11275 Remove some uses of integer literals in oplog code.
SERVER-11295 Encode 'anyAction' in a future-proof way
SERVER-11178: Introduce IndexCatalog and requirements
IndexCatalog will be the only place to get information about indexes, and modify them
still a lot of work left, but a lot of the logic is now shifted
SERVER-11214 drop_sharded_db.js (sharding_auth) failed on Linux 64-bit DEBUG

Fix race in ClientCursor::invalidate
SERVER-8864 Fixed ssl/replset1.js test
SERVER-10830 use $WIX and $MERGEMODULESBASEPATH for enterprise msi