Fixed a slow id generator rebuild high id issue

where the high id would be set too high.
Also removes a test that, using complicated breakpoint testing, basically
tested that IdGeneratorImpl#setHighId throws exception when given -1 as
high Id. Replaced with a unit test.
Fix formatting of the description of the mapped_memory_total_size configuration
Fixes an issue where id generator picked up freeId calls during recovery

where it shouldn't have done so. It's due to a recent change removing the
recovery flag for the stores. Now setStoreNotOk() sets idGenerator null so
that even if the id generator was opened cleanly, but later we found out
that the store needed recovery we would clear it so that the id generator
wouldn't pick up freeId calls during recovery.
Remove metadata/id from the representation

Pending discussion on its format
ExpandPipe should use relation type indices when possible

This change lets ExpandPipe use relationships ids instead of names whenever all ids in question are resolved.
Fixes a race where multiple threads were writing to the same  channel

and they all called channel.position(position) / channel.write(data),
whereas they now call channel.write(data, position)
Return null instead of throwing SchemaRuleNotFoundException when index not found
Enabled parallel I/O writing in test to try to expose problem

that has been previously seen where some pages wasn't written properly.
VarLengthExpandPipe uses token ids whenever possible.

When all relationships tokens are resolved VarLengthExpandPipe
uses these instead of using the name.
Merge pull request #3151 from cleishm/metadata_no_id

[2.1] Remove metadata/id from the representation
Tweak selectivity and costs

* Improved equality comparison selectivity
* Cost model now separates operators between IO and CPU bound
* Tweaked default index selectivity
Merge pull request #3146 from tinwelint/2.2-freeid-during-recovery-fix

[2.2] Fixes an issue where id generator picked up freeId calls during recovery
Merge branch '2.1-maint'
Merge pull request #3142 from tinwelint/2.2-too-high-id-after-slow-rebuild

[2.2] Fixed a slow id generator rebuild high id issue
Merge pull request #3147 from chrisvest/2.2-pagecache-docs

[2.2] Fix formatting of the description of the mapped_memory_total_size configuration
Merge pull request #3155 from lutovich/2.1-no-exceptions-for-flow-control

[2.1] Return null instead of throwing SchemaRuleNotFoundException when index not found
Merge branch '2.1-maint'

Conflicts:
	community/kernel/src/main/java/org/neo4j/kernel/impl/api/StateHandlingStatementOperations.java
Merge pull request #3153 from pontusmelke/expand-pipe-using-ids

[2.2] ExpandPipe and VarLengthExpandPipe should use relation type token id when possible
Merge pull request #3156 from systay/cost-tweaks

[2.2] Tweak selectivity and costs
TODO fixes