New Rule S2445: Blocks should be synchronized on read-only fields

[zsolt-kolbay-sonarsource]

Implement S2445 for C#

The check for private has been removed, following the outcome of the discussion here, summarized here.

[pavel-mikula-sonarsource]

We need to rename the rule to C# syntax

[antonioaversa]

Results of Peach Validation: many FP found on "locking of local variables" for "advanced" synchronization scenarios such as:

• fine graned synchronization via collections of lock objects;
• renaming of a readonly field in the context of the current method;
• locking inside a loop, on the iteration variable.

A new rule S6507 has been created (#6854), to extract the offending check from S2445.
S6507 is still a BUG, but it is not in the Sonar Way.