ensure upserts occur in READ_COMMITTED isolation level

[uladkasach]

REPEATABLE_READ is not required for insert-only upserts (since we only check existence and insert rows for static entities, and only update the "current_version_pointer" table for dynamic entities - but that should not be a problem either).

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Why do we want to do this? because otherwise REPEATABLE_READ results in deadlocks in high concurrency usage:

------------------------
LATEST DETECTED DEADLOCK
------------------------
2020-01-19 14:28:28 0x7fdb08c5b700
*** (1) TRANSACTION:
TRANSACTION 1886, ACTIVE 0 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 3 lock struct(s), heap size 1136, 2 row lock(s), undo log entries 1
MySQL thread id 10, OS thread handle 140578722281216, query id 205 172.19.0.1 svc_idea_curate update
INSERT INTO tag
LOCK WAIT 3 lock struct(s), heap size 1136, 2 row lock(s), undo log entries 1
MySQL thread id 10, OS thread handle 140578722281216, query id 205 172.19.0.1 svc_idea_curate update
INSERT INTO tag
      (uuid, name)
      VALUES
      (uuid(),  NAME_CONST('in_name',_utf8mb4'outdoors' COLLATE 'utf8mb4_unicode_ci'))
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 32 page no 4 n bits 72 index tag_ux1 of table `ideacurationdb`.`tag` trx id 1886 lock_mode X locks gap before rec insert intention waiting
Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
 0: len 5; hex 7269766572; asc river;;
 1: len 8; hex 8000000000000002; asc         ;;

*** (2) TRANSACTION:
TRANSACTION 1887, ACTIVE 0 sec inserting
mysql tables in use 1, locked 1
3 lock struct(s), heap size 1136, 2 row lock(s), undo log entries 1
MySQL thread id 9, OS thread handle 140578721740544, query id 207 172.19.0.1 svc_idea_curate update
INSERT INTO tag
      (uuid, name)
      VALUES
      (uuid(),  NAME_CONST('in_name',_utf8mb4'park' COLLATE 'utf8mb4_unicode_ci'))
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 32 page no 4 n bits 72 index tag_ux1 of table `ideacurationdb`.`tag` trx id 1887 lock_mode X locks gap before rec
Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
 0: len 5; hex 7269766572; asc river;;
 1: len 8; hex 8000000000000002; asc         ;;

*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 32 page no 4 n bits 72 index tag_ux1 of table `ideacurationdb`.`tag` trx id 1887 lock_mode X locks gap before rec insert intention waiting
Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
 0: len 5; hex 7269766572; asc river;;
 1: len 8; hex 8000000000000002; asc         ;;

*** WE ROLL BACK TRANSACTION (2)

We were upserting two static entities, and they locked eachother because, it looks like, they were using a "gap" lock and affected the same gap. See https://dba.stackexchange.com/questions/202455/mysql-gap-lock-deadlock-on-inserts for more details