session: make `CurrentBootstrapVersion` exported

By design, the tool BR(Backup/Restore) integrates tidb-server, and it needs
this varible to make sure the bootstrap version of the tidb-server it
integrated is the same with the cluster it serves.

store/tikv: make LockKeys() function thread safe (pingcap#6340) (ping…

…cap#6384)

[release-1.0] cherry pick add manual gc (pingcap#6326)

ddl: support the rolling upgrade. (pingcap#6301) (pingcap#6305)

Merge master to 2.0 to release rc5 (pingcap#6288)

plan: statsInfo.count for LogicalTableDual should be its RowCount (pi…

…ngcap#6190)

executor: remove Next function for HashAggExec (pingcap#5987)

executor: remove Next for HashJoinExec (pingcap#5991)

*: move irrelevant code out of package "distsql" (pingcap#5893)

parser: create index support LOCK syntax (pingcap#5851) (pingcap#5853)