MERGE target hints support

[MaceWindu]

Fix #1272

SQL Server supports hints on target table, this PR adds support for hints generation.

Because merge uses different ordering of hint and table alias, you cannot use With method, which will put hint after alias and need to use WithTableExpression method to specify proper ordering:

// {0} is a table
// {1} is an alias
// for merge target alias must go after hint
table.WithTableExpression("{0} WITH (HOLDLOCK) {1}")

TODO:

• [ ] UsingTarget method produce bad sql. We should remove hint from target table, when it used as source

Edit:
code above shouldn't be used for merge-specific hints, see below

[MaceWindu]

Taking into account

• ITable<T> extension methods should return new ITable instance instead of modified instance, passed to them #1274
• target hint having custom syntax
• table hints doesn't work well with UsingTarget API

I will add merge hint parameter of merge operation that user could pass to Merge/MergeInto methods.
This will allow us to generate hints in proper places easily for all databases that support merge-specific hints.

[MaceWindu]

Ok, what we have. I've added overloads to Merge/MergeInto methods to accept string hint. Those overloads will accept only non-null values and execution of merge will throw exception if provider doesn't support hints.

What providers supported (only those have hints according to documentation)

Sql Server

Hint applied to merge target (OPTIONS clause still not supported). Example:

db.TargetTable
    .Merge("HOLDLOCK")
    .UsingTarget()
    .OnTargetKey()
    .UpdateWhenMatched()
    .Merge();

MERGE INTO [TargetTable] WITH(HOLDLOCK) [Target] ....

Oracle

db.TargetTable
    .Merge("append")
    .UsingTarget()
    .OnTargetKey()
    .UpdateWhenMatched()
    .Merge();

MERGE /*+ append */ INTO TestMerge1 Target ...

Informix

db.TargetTable
    .Merge("AVOID_STMT_CACHE")
    .UsingTarget()
    .OnTargetKey()
    .UpdateWhenMatched()
    .Merge();

MERGE {+ AVOID_STMT_CACHE } INTO TestMerge1 Target ...