remove reduntant item
Merge branch 'Client & Server'

Conflicts:
	.gitignore
try to merge the data type codes contributed by dsc
data type promotion
a synchronizing point for zhuhe and kai
+ for arithmetic expression, < for compare expression,
CASE..WHEN..THEN..ELSE for composite function.
test utility
add setValue methods to ExpressionItem to facilite the intialization.
3-8
3-9
3-9 20.31
test for push
Expandable framework seems to work~ -_-.
Fixed a bug in aggregation caused by partially lock.
add proper const property to some methods.
Add property const properties to some methods.
support lock and unlock on a given hash bucket.
Add context structure and related access methods.
Currently, the context contains the block for asking and its iterator.
Expanded thread can be created and dropped on demands. Context has been
integrated into Filter.
Expandable Scan and Filter currently well support threading expanding
and thread shrinking